[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"minerals:one:6666":3},{"id":4,"longid":5,"guid":6,"name":7,"shortcode_ima":8,"entrytype":9,"entrytype_text":10,"varietyof":11,"synid":8,"polytypeof":8,"groupid":8,"weighting":12,"nolocadd":13,"blacklisted":13,"mindat_formula":14,"mindat_formula_note":8,"ima_formula":8,"elements":15,"sigelements":19,"key_elements":8,"impurities":8,"cim":8,"ima_status":8,"ima_notes":8,"ima_history":8,"approval_year":8,"publication_year":8,"discovery_year":8,"strunz10ed1":20,"strunz10ed2":20,"strunz10ed3":20,"strunz10ed4":8,"dana8ed1":20,"dana8ed2":20,"dana8ed3":20,"dana8ed4":20,"csystem":8,"cclass":8,"spacegroup":8,"spacegroupset":20,"a":8,"b":8,"c":8,"alpha":8,"beta":8,"gamma":8,"aerror":8,"berror":8,"cerror":8,"alphaerror":8,"betaerror":8,"gammaerror":8,"va3":8,"z":8,"csmetamict":13,"commentcrystal":8,"twinning":8,"tranglide":8,"parting":8,"epitaxidescription":8,"morphology":8,"tlform":8,"hmin":21,"hmax":22,"hardtype":8,"vhnmin":20,"vhnmax":20,"vhnerror":8,"vhng":8,"vhns":8,"commenthard":8,"dmeas":20,"dmeas2":20,"dcalc":20,"dmeaserror":8,"dcalcerror":8,"commentdense":8,"lustre":8,"lustretype":23,"commentluster":8,"diapheny":24,"streak":25,"colour":26,"commentcolor":8,"colors":8,"streak_colors":27,"luminescence":8,"uv":8,"cleavage":8,"cleavagetype":8,"fracturetype":28,"tenacity":8,"commentbreak":8,"opticaltype":8,"opticalsign":8,"opticalalpha":8,"opticalalpha2":20,"opticalalphaerror":8,"opticalbeta":8,"opticalbeta2":20,"opticalbetaerror":8,"opticalgamma":8,"opticalgamma2":20,"opticalgammaerror":8,"opticalomega":8,"opticalomega2":20,"opticalomegaerror":8,"opticalepsilon":8,"opticalepsilon2":20,"opticalepsilonerror":8,"opticaln":8,"opticaln2":8,"opticalnerror":8,"optical2vcalc":8,"optical2vcalc2":8,"optical2vcalcerror":8,"optical2vmeasured":8,"optical2vmeasured2":8,"optical2vmeasurederror":8,"rimin":8,"rimax":8,"opticaldispersion":8,"opticalpleochroism":8,"opticalpleochorismdesc":8,"opticalbirefringence":8,"opticalcomments":8,"opticalcolour":8,"opticalinternal":8,"opticaltropic":8,"opticalanisotropism":8,"opticalbireflectance":8,"opticalextinction":8,"opticalr":8,"specdispm":8,"ir":8,"electrical":8,"magnetism":8,"thermalbehaviour":8,"other":8,"industrial":8,"occurrence":8,"otheroccurrence":8,"type_specimen_store":8,"description_short":8,"aboutname":29,"rock_parent":8,"rock_parent2":8,"rock_root":30,"rock_bgs_code":8,"meteoritical_code":8,"updttime":31,"reviewed_at":8,"variety_of":32,"varieties":38,"group_members":58,"associates":59,"confused_with":60,"type_localities":61,"occurrence_total":62,"citations":63,"images":121,"structures":568,"synonyms":569,"language_names":571,"wikidata_qid":8,"texts":572},6666,"1:1:6666:8","73b847cc-22db-4ea7-ba1d-1e0844734972","Precious Opal",null,2,"variety",3004,4258,false,"SiO\u003Csub>2\u003C\u002Fsub>·nH\u003Csub>2\u003C\u002Fsub>O",[16,17,18],"Si","O","H",[16,17,18],"0",5.5,6.5,"Vitreous,Sub-Vitreous,Greasy","Translucent","white","Multi-coloured",[25],"Conchoidal","The origin of the word \"opal\" is uncertain. It may be from the Sanskrit \"upala\", meaning \"stone\" or \"precious stone\" or from opalus, the ancient Latin name for the gem (Pliny the Elder, 75-79). Pliny may have also referred to the gem as paederos, but a modern commentary by Kostov (2008) questions if that name was actually applied to the opal in the modern sense.",0,"2025-08-11 12:14:27",{"id":11,"name":33,"entrytype":30,"csystem":8,"ima_formula":34,"mindat_formula":14,"hmin":21,"hmax":22,"dmeas":35,"dcalc":20,"strunz10ed1":36,"primary_image_id":37},"Opal","SiO\u003Csub>2\u003C\u002Fsub> · nH\u003Csub>2\u003C\u002Fsub>O","1.9","4",18161,[39,42,46,50,54],{"id":40,"name":41,"entrytype":9,"csystem":8,"ima_formula":8,"mindat_formula":14,"hmin":8,"hmax":8,"dmeas":20,"dcalc":20,"primary_image_id":8},7998,"Black Opal",{"id":43,"name":44,"entrytype":9,"csystem":8,"ima_formula":8,"mindat_formula":14,"hmin":8,"hmax":8,"dmeas":20,"dcalc":20,"primary_image_id":45},8000,"Boulder Opal",36683,{"id":47,"name":48,"entrytype":9,"csystem":8,"ima_formula":8,"mindat_formula":14,"hmin":8,"hmax":8,"dmeas":20,"dcalc":8,"primary_image_id":49},8003,"Crystal Opal",51243,{"id":51,"name":52,"entrytype":9,"csystem":8,"ima_formula":8,"mindat_formula":8,"hmin":8,"hmax":8,"dmeas":20,"dcalc":8,"primary_image_id":53},9807,"Opal Matrix",71531,{"id":55,"name":56,"entrytype":9,"csystem":8,"ima_formula":8,"mindat_formula":8,"hmin":8,"hmax":8,"dmeas":20,"dcalc":8,"primary_image_id":57},9836,"White Opal",85998,[],[],[],[],104,[64,69,73,78,83,87,92,96,100,104,108,112,117],{"id":65,"year":66,"html":67,"doi":68},2522392,1964,"JONES, J. B., SANDERS, J. V., SEGNIT, E. R. (1964) Structure of Opal. \u003Ci>Nature\u003C\u002Fi>, 204 (4962). 990-991 \u003Ca target='_blank' href='https:\u002F\u002Fdoi.org\u002F10.1038\u002F204990a0'>doi:10.1038\u002F204990a0\u003C\u002Fa>","10.1038\u002F204990a0",{"id":70,"year":66,"html":71,"doi":72},2522986,"SANDERS, J. V. (1964) Colour of Precious Opal. \u003Ci>Nature\u003C\u002Fi>, 204 (4964). 1151-1153 \u003Ca target='_blank' href='https:\u002F\u002Fdoi.org\u002F10.1038\u002F2041151a0'>doi:10.1038\u002F2041151a0\u003C\u002Fa>","10.1038\u002F2041151a0",{"id":74,"year":75,"html":76,"doi":77},2534408,1966,"DARRAGH, P. J., GASKIN, A. J., TERRELL, B. C., SANDERS, J. V. (1966) Origin of Precious Opal. \u003Ci>Nature\u003C\u002Fi>, 209 (5018). 13-16 \u003Ca target='_blank' href='https:\u002F\u002Fdoi.org\u002F10.1038\u002F209013a0'>doi:10.1038\u002F209013a0\u003C\u002Fa>","10.1038\u002F209013a0",{"id":79,"year":80,"html":81,"doi":82},565312,1971,"Jones, J. B., Segnit, E. R. (1971) The nature of opal I. nomenclature and constituent phases. \u003Ci>Journal of the Geological Society of Australia\u003C\u002Fi>,  18 (1) 57-68 \u003Ca target='_blank' href='https:\u002F\u002Fdoi.org\u002F10.1080\u002F00167617108728743'>doi:10.1080\u002F00167617108728743\u003C\u002Fa>","10.1080\u002F00167617108728743",{"id":84,"year":85,"html":86,"doi":8},15966567,1975,"Sanders, J. V. (1975). Microstructure and crystallinity of gem opals. American Mineralogist, 60, 749-757.",{"id":88,"year":89,"html":90,"doi":91},3235954,1994,"Elzea, J.M., Odom, I.E., Miles, W.J. (1994) Distinguishing well ordered opal-CT and opal-C from high temperature cristobalite by x-ray diffraction. \u003Ci>Analytica Chimica Acta\u003C\u002Fi>, 286 (1). 107-116 \u003Ca target='_blank' href='https:\u002F\u002Fdoi.org\u002F10.1016\u002F0003-2670(94)80182-7'>doi:10.1016\u002F0003-2670(94)80182-7\u003C\u002Fa>","10.1016\u002F0003-2670(94)80182-7",{"id":93,"year":94,"html":95,"doi":8},16129728,1996,"Lapis Extra No. 10, Opal (1996).",{"id":97,"year":98,"html":99,"doi":8},16119011,2008,"Kostov, Rusian I. (2008) Orphic Lithica As A Source Of Late Antiquity Mineralogical Knowledge. \u003Ci>Annual Of The University Of Mining And Geology “ST. Ivan Rilski”\u003C\u002Fi>,  51 (1) 109-115",{"id":101,"year":98,"html":102,"doi":103},395937,"Gaillou, E., Fritsch, E., Aguilar-Reyes, B., Rondeau, B., Post, J., Barreau, A., Ostroumov, M. (2008) Common gem opal: An investigation of micro- to nano-structure. \u003Ci>American Mineralogist\u003C\u002Fi>,  93 (11) 1865-1873 \u003Ca target='_blank' href='https:\u002F\u002Fdoi.org\u002F10.2138\u002Fam.2008.2518'>doi:10.2138\u002Fam.2008.2518\u003C\u002Fa>","10.2138\u002Fam.2008.2518",{"id":105,"year":106,"html":107,"doi":8},15954047,2018,"Liesegang, Moritz, and Ralf Milke. (2018) \"Silica Colloid Ordering in a Dynamic Sedimentary Environment\" Minerals 8, no. 1: 12. https:\u002F\u002Fdoi.org\u002F10.3390\u002Fmin8010012",{"id":109,"year":110,"html":111,"doi":8},16130063,2019,"Pecover, S.R. (2019): InColor 41, 34-60. [https:\u002F\u002Fwww.gemstone.org\u002Fincolor\u002F41\u002F54\u002F]",{"id":113,"year":114,"html":115,"doi":116},16948225,2023,"Mustoe, George E., Smith, Elizabeth T. (2023) Timing of Opalization at Lightning Ridge, Australia: New Evidence from Opalized Fossils. \u003Ci>Minerals\u003C\u002Fi>,  13 (12)  \u003Ca target='_blank' href='https:\u002F\u002Fdoi.org\u002F10.3390\u002Fmin13121471'>doi:10.3390\u002Fmin13121471\u003C\u002Fa>","10.3390\u002Fmin13121471",{"id":118,"year":114,"html":119,"doi":120},15738507,"Chauviré, Boris, Mollé, Valentin, Guichard, Florine, Rondeau, Benjamin, Thomas, Paul Stephen, Fritsch, Emmanuel (2023) Cracking of Gem Opals. \u003Ci>Minerals\u003C\u002Fi>, 13 (3) 356 \u003Ca target='_blank' href='https:\u002F\u002Fdoi.org\u002F10.3390\u002Fmin13030356'>doi:10.3390\u002Fmin13030356\u003C\u002Fa> \u003Ca target='_blank' href='https:\u002F\u002Fwww.mdpi.com\u002F2075-163X\u002F13\u002F3\u002F356\u002Fpdf?version=1677826705' class='refpdflink'>\u003C\u002Fa>","10.3390\u002Fmin13030356",[122,132,142,150,158,166,174,182,190,198,205,212,219,226,233,240,247,255,263,271,279,286,294,302,309,315,322,329,335,342,350,358,366,373,380,387,395,403,410,417,425,432,440,447,455,463,471,479,487,494,502,509,516,523,530,537,544,550,559],{"id":123,"source_url":124,"license_code":125,"credit_html":126,"title":127,"description":128,"author":129,"original_width":130,"original_height":131},75294,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=10171663","CC BY-SA 3.0","Robert M. Lavinsky, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=10171663\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Opal-275143.jpg","\u003Ca href=\"https:\u002F\u002Fen.wikipedia.org\u002Fwiki\u002FOpal\" class=\"extiw\" title=\"en:Opal\">Opal\u003C\u002Fa> (Var.: Precious Opal)\n\u003Cdl>\u003Cdd>\u003Cdl>\u003Cdd>Locality: Gracios O Dios Mine, Honduras (\u003Ca rel=\"nofollow\" class=\"external text\" href=\"http:\u002F\u002Fwww.mindat.org\u002Floc-11444.html\">Locality at mindat.org\u003C\u002Fa>)\u003C\u002Fdd>\n\u003Cdd>Size: 1.7 x 1.6 x 1.0 cm.\u003C\u002Fdd>\n\u003Cdd>Opal is one of those minerals that look as beautiful and impressive when it is opaque as when it is water clear. The amazing range of this material (considering the simplistic chemistry) makes it an interesting mineral, and that's saying a lot considering the species does not even have any crystal form. Recently, I obtained a small parcel of precious Opal from Honduras, The Opal from this Gracious O Dios Mine have been mined for over 150 years, and have proven to be some of the more attractive yet lesser known from the Western Hemisphere. This piece features a very attractive little patch of multicolor Opal on matrix which shows flashes of red, gold, orange, blue and green. Ex. Brian Kosnar Collection.\u003C\u002Fdd>\u003C\u002Fdl>\u003C\u002Fdd>\u003C\u002Fdl>","Robert M. Lavinsky",503,432,{"id":133,"source_url":134,"license_code":135,"credit_html":136,"title":137,"description":138,"author":139,"original_width":140,"original_height":141},75295,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=35318491","CC BY 2.0","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=35318491\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Opalized fossil bivalve, Coober Pedy Opal Field, South Australia.jpg","\u003Cp>Opalized bivalve from the Coober Pedy Opal Field, South Australia.\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>The Coober Pedy area of southern Australia is world-famous for its high-quality precious opal (= hydrous silica - SiO2·nH2O).  Some of the opal at Coober Pedy (pronounced “koober peedee”) has replaced fossil skeletal material.  Most of the opalized fossils here are various marine bivalve (clam) species, but Coober Pedy opal replaces other fossil remains as well (for example, snails, belemnites, crinoids, ichthyosaurs, and plesiosaurs).  Even soft parts have been preserved & replaced by opal (Kear, 2006).\n\u003C\u002Fp>\u003Cp>The specimen shown here is a polished fossil clam that may be assignable to Cyrenopsis (?) (Animalia, Mollusca, Bivalvia, Heterodonta, Veneroida, Arcticaceae, Neomiodontidae).\n\u003C\u002Fp>\u003Cp>Locality: unrecorded mine field in Coober Pedy Opal Field, north-central South Australia State, southern Australia.\n\u003C\u002Fp>\u003Cp>Stratigraphy: “weathered zone” of the Bulldog Shale, lower Marree Subgroup, Aptian Stage, upper Lower Cretaceous.\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nKear, B.P.  2006.  Marine reptiles from the Lower Cretaceous of South Australia: elements of a high-latitude cold-water assemblage.  Palaeontology 49: 837-856.","James St. John",2776,1776,{"id":143,"source_url":144,"license_code":135,"credit_html":145,"title":146,"description":147,"author":139,"original_width":148,"original_height":149},50331,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500420","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500420\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal in matrix (Quilpe, Queensland, Australia) 2 (26554894833).jpg","\u003Cp>Precious opal in matrix from Australia. (Stan Woollams & Pris Woollams collection)\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids - <a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\">https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\u003C\u002Fa>\" rel=\"nofollow\">www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg<\u002Fa>) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>Locality: unrecorded site at or near Quilpe, southern Queensland, eastern Australia\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",2469,1388,{"id":151,"source_url":152,"license_code":135,"credit_html":153,"title":154,"description":155,"author":139,"original_width":156,"original_height":157},50332,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500424","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500424\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Delanta District, South Wello, Ethiopia) 2 (29802309635).jpg","\u003Cp>Precious opal from the Tertiary of Ethiopia.\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids - <a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\">https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\u003C\u002Fa>\" rel=\"nofollow\">www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg<\u002Fa>) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>The Ethiopian precious opal shown above is a nodule from a Tertiary-aged volcanic tuff (\"ignimbrite\").\n\u003C\u002Fp>\u003Cp>Locality: unrecorded locality in Delanta District, South Wello (South Wollo; South Welo), southeastern Amhara Province, north-central Ethiopia, eastern Africa\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",3460,2868,{"id":159,"source_url":160,"license_code":135,"credit_html":161,"title":162,"description":163,"author":139,"original_width":164,"original_height":165},71547,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500492","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500492\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Queensland, Australia) 3 (29804291111).jpg","\u003Cp>Precious opal from Australia. (CIS 212-152, Cranbrook Institute of Science collection, Bloomfield Hills, Michigan, USA)\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids - <a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\">https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\u003C\u002Fa>\" rel=\"nofollow\">www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg<\u002Fa>) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>Locality: unrecorded locality in Queensland, Australia\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>References cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",1882,1928,{"id":167,"source_url":168,"license_code":135,"credit_html":169,"title":170,"description":171,"author":139,"original_width":172,"original_height":173},50333,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500517","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500517\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Quilpie, Queensland, Australia) 2 (29617153490).jpg","\u003Cp>Precious opal from Australia. (public display, Denver Museum of Nature & Science, Denver, Colorado, USA)\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 5100 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids - <a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\">https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\u003C\u002Fa>\" rel=\"nofollow\">www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg<\u002Fa>) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>Locality: unrecorded site in the Quilpie area, Queensland, Australia\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>References cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",2814,2966,{"id":175,"source_url":176,"license_code":135,"credit_html":177,"title":178,"description":179,"author":139,"original_width":180,"original_height":181},50334,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84625649","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84625649\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Tertiary; Shewa Province, Ethiopia) 12 (48193707012).jpg","\u003Cp>Precious opal from the Tertiary of Ethiopia.\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 5400 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres called \"colloids\" that can be seen with a scanning electron microscope (SEM). (<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.geo.arizona.edu\u002Fxtal\u002Fgeos306\u002Fopal-spheres.jpg\">https:\u002F\u002Fwww.geo.arizona.edu\u002Fxtal\u002Fgeos306\u002Fopal-spheres.jpg\u003C\u002Fa>\" rel=\"noreferrer nofollow\">www.geo.arizona.edu\u002Fxtal\u002Fgeos306\u002Fopal-spheres.jpg<\u002Fa>)\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>Locality: unrecorded\u002Fundisclosed locality in Shewa Province, Ethiopia, eastern Africa\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"noreferrer nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",2753,1397,{"id":183,"source_url":184,"license_code":135,"credit_html":185,"title":186,"description":187,"author":139,"original_width":188,"original_height":189},18164,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=80125759","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=80125759\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Coober Pedy Opal Field, South Australia).jpg","Precious opal (Coober Pedy Opal Field, South Australia) \n\u003Cp>Opal is hydrous silica (SiO2·nH2O). Technically, opal is not a mineral because it lacks a crystalline structure. Opal is supposed to be called a mineraloid. Opal is made up of extremely tiny spheres (colloids - www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence). This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids. Different opalescent colors are produced by colloids of differing sizes. If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced. Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however. Common opal has a wax-like luster & is often milky whitish with no visible color play at all. Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians. Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass. Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\nLocality: unrecorded\u002Fundisclosed site in the Coober Pedy Opal Field, north-central South Australia State, southern Australia",1011,1480,{"id":191,"source_url":192,"license_code":135,"credit_html":193,"title":194,"description":195,"author":139,"original_width":196,"original_height":197},37665,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198570","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198570\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Tertiary; Ethiopia) 4.jpg","Precious opal from the Tertiary of Ethiopia.\n\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 6000 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>The specimen seen here is opal-CT, which consists of extremely tiny cristobalite-tridymite aggregates called leptospheres.  This is in contrast to Australian precious opal, which is opal-A (= consists of amorphous, hydrous silica colloids).  Ethiopian precious opal occurs as nodules in Tertiary-aged volcanic tuffs.\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\nwww.mindat.org\u002Fgallery.php?min=3004\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",3996,2482,{"id":199,"source_url":200,"license_code":135,"credit_html":201,"title":202,"description":195,"author":139,"original_width":203,"original_height":204},37667,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198572","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198572\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Tertiary; Ethiopia) 3.jpg",3830,2487,{"id":206,"source_url":207,"license_code":135,"credit_html":208,"title":209,"description":195,"author":139,"original_width":210,"original_height":211},37668,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198578","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198578\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Tertiary; Ethiopia) 8.jpg",3714,2616,{"id":213,"source_url":214,"license_code":135,"credit_html":215,"title":216,"description":195,"author":139,"original_width":217,"original_height":218},37669,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198581","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198581\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Tertiary; Ethiopia) 9.jpg",3640,2444,{"id":220,"source_url":221,"license_code":135,"credit_html":222,"title":223,"description":195,"author":139,"original_width":224,"original_height":225},37670,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198588","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198588\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Tertiary; Ethiopia) 14.jpg",1309,1382,{"id":227,"source_url":228,"license_code":135,"credit_html":229,"title":230,"description":195,"author":139,"original_width":231,"original_height":232},37672,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198590","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198590\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Tertiary; Ethiopia) 15.jpg",1367,1038,{"id":234,"source_url":235,"license_code":135,"credit_html":236,"title":237,"description":195,"author":139,"original_width":238,"original_height":239},37673,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198593","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198593\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Tertiary; Ethiopia) 17.jpg",1697,1105,{"id":241,"source_url":242,"license_code":135,"credit_html":243,"title":244,"description":195,"author":139,"original_width":245,"original_height":246},37674,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198596","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198596\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Tertiary; Ethiopia) 18.jpg",1671,1247,{"id":248,"source_url":249,"license_code":135,"credit_html":250,"title":251,"description":252,"author":139,"original_width":253,"original_height":254},71548,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500518","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500518\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Andamooka Opal Fields, South Australia) 4 (29307225203).jpg","\u003Cp>Precious opal from Australia. (public display, Denver Museum of Nature & Science, Denver, Colorado, USA)\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 5100 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids - <a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\">https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\u003C\u002Fa>\" rel=\"nofollow\">www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg<\u002Fa>) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>Locality: unrecorded locality in the Andamooka Opal Fields, ~eastern edge of the Stuart Range Plateau, west of northern Lake Torrens, southeast-central South Australia (environs of 30° 26' 50\" South latitude, 137° 09' 55\" East longitude)\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",3350,2748,{"id":256,"source_url":257,"license_code":135,"credit_html":258,"title":259,"description":260,"author":139,"original_width":261,"original_height":262},71626,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500535","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500535\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Shewa Province, Ethiopia) 7 (30159429235).jpg","\u003Cp>Precious opal from the Tertiary of Ethiopia. (1.5 centimeters across at its widest)\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 5400 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids - <a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\">https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\u003C\u002Fa>\" rel=\"noreferrer nofollow\">www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg<\u002Fa>) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>The specimen shown above is opal-CT, which consists of extremely tiny cristobalite-tridymite aggregates called leptospheres.  This is in contrast to Australian precious opal, which is opal-A (= consists of amorphous, hydrous silica colloids).  This Ethiopian precious opal is from a nodule in a Miocene-aged rhyolitic welded tuff (\"ignimbrite\").\n\u003C\u002Fp>\u003Cp>Stratigraphy: Alaji Rhyolite Formation (a.k.a. Amba Alaji Rhyolites; a.k.a. Aliyu Amba Ignimbrites), Lower to Middle Miocene\n\u003C\u002Fp>\u003Cp>Locality: attributed to Shewa Province (Shoa Province) (if accurate, the specimen is possibly from the northern flank of Yita Ridge or from the Guy Meda Wihe River drainage basin or from the southern flank of Gift Ridge, ~12 miles north of Mezezo & ~150 miles northeast of Addis Ababa, west of the Great Rift Valley of Ethiopia (Main Ethiopian Rift), Menz Gishe District, northeastern Shewa Province), northeast-central Ethiopia, eastern Africa\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"noreferrer nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",1728,1272,{"id":264,"source_url":265,"license_code":135,"credit_html":266,"title":267,"description":268,"author":139,"original_width":269,"original_height":270},71627,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500538","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500538\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Shewa Province, Ethiopia) 9 (30159429085).jpg","\u003Cp>Precious opal from the Tertiary of Ethiopia. (1.6 centimeters across at its widest)\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 5400 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids - <a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\">https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\u003C\u002Fa>\" rel=\"noreferrer nofollow\">www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg<\u002Fa>) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>The specimen shown above is opal-CT, which consists of extremely tiny cristobalite-tridymite aggregates called leptospheres.  This is in contrast to Australian precious opal, which is opal-A (= consists of amorphous, hydrous silica colloids).  This Ethiopian precious opal is from a nodule in a Miocene-aged rhyolitic welded tuff (\"ignimbrite\").\n\u003C\u002Fp>\u003Cp>Stratigraphy: Alaji Rhyolite Formation (a.k.a. Amba Alaji Rhyolites; a.k.a. Aliyu Amba Ignimbrites), Lower to Middle Miocene\n\u003C\u002Fp>\u003Cp>Locality: attributed to Shewa Province (Shoa Province) (if accurate, the specimen is possibly from the northern flank of Yita Ridge or from the Guy Meda Wihe River drainage basin or from the southern flank of Gift Ridge, ~12 miles north of Mezezo & ~150 miles northeast of Addis Ababa, west of the Great Rift Valley of Ethiopia (Main Ethiopian Rift), Menz Gishe District, northeastern Shewa Province), northeast-central Ethiopia, eastern Africa\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"noreferrer nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",1656,1294,{"id":272,"source_url":273,"license_code":135,"credit_html":274,"title":275,"description":276,"author":139,"original_width":277,"original_height":278},71628,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500543","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500543\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Tertiary; Ethiopia) 2 (31261559240).jpg","\u003Cp>Precious opal from the Tertiary of Ethiopia. (photo by Ljubomir Ristestki)\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids - <a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\">https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\u003C\u002Fa>\" rel=\"nofollow\">www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg<\u002Fa>) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>The specimen shown above is opal-CT, which consists of extremely tiny cristobalite-tridymite aggregates called leptospheres.  This is in contrast to Australian precious opal, which is opal-A (= consists of amorphous, hydrous silica colloids).  Ethiopian precious opal occurs as nodules in Tertiary-aged volcanic tuffs.\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",1096,1141,{"id":280,"source_url":281,"license_code":282,"credit_html":283,"title":33,"description":8,"author":8,"original_width":284,"original_height":285},89091,"https:\u002F\u002Fgeocollections.info\u002Ffile\u002F61183","CC BY-SA 4.0","Photo: Unknown author — http:\u002F\u002Fcreativecommons.org\u002Flicenses\u002Fby-sa\u002F4.0\u002F, courtesy of \u003Ca href=\"https:\u002F\u002Fgeocollections.info\u002Ffile\u002F61183\" rel=\"noopener\">The Estonian Museum of Natural History\u003C\u002Fa> via Europeana",1000,750,{"id":287,"source_url":288,"license_code":135,"credit_html":289,"title":290,"description":291,"author":139,"original_width":292,"original_height":293},50335,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=85906080","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=85906080\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Opal in hydrothermally-altered rhyolitic obsidian (Tertiary; mine at Opal Mountain, Clark County, Idaho, USA) 7.jpg","Opal in rhyolitic obsidian from the Tertiary of Idaho, USA.\n\u003Cp>This rock is on display at a precious opal pay site in the town of Spencer, located on the eastern side of Interstate 15, eastern Idaho.  Rocks are quarried at a nearby mountain and dumped here.  The material is composed of Early Pliocene rhyolitic volcanics that were erupted from the Heise Volcanic Center.  Observed lithologies range from rhyolite to rhyolitic obsidian to hydrothermally altered versions.  The obsidian varies: black spherulitic obsidian, brown spherulitic obsidian, flow-banded spherulitic obsidian.  Opal (= SiO2·nH2O - hydrous silica) occurs in these rocks - it is secondary (early secondary?) and hydrothermal in origin.  The opal fills cavities and lithophysae in the host rocks.  Observed opal varieties here include common opal (milky opal; = opalite, which is opal rock), hyalite opal, and precious opal.  Also noted were turgite on hyalite and drusy quartz.\n\u003C\u002Fp>\u003Cp>Stratigraphy: possibly derived from a lava flow called the Opal Mountain Rhyolite, Kilgore Tuff, upper Heise Group, Lower Pliocene, 4.45 Ma\n\u003C\u002Fp>\nLocality: derived from an opal mine on the southern side of Opal Mountain between Rattlesnake Creek & Three Mile Creek, ENE of Spencer, north-central Clark County, north of the eastern Snake River Plain, Idaho, USA (44° 22’ 43.94” North latitude, 112° 05’ 38.13” West longitude)",3008,2000,{"id":295,"source_url":296,"license_code":135,"credit_html":297,"title":298,"description":299,"author":139,"original_width":300,"original_height":301},50338,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=126617125","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=126617125\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal in basalt (Honduras) 7.jpg","Precious opal (\"black matrix opal\") in Tertiary-aged basalt from Honduras.\n\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are about 5800 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>Locality: unrecorded \u002F undisclosed site in Honduras (possibly - probably? - from theTablon Mine, near Erandique, southeastern Lempira Department, western Honduras)\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\nwww.mindat.org\u002Fgallery.php?min=3004\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",1265,1450,{"id":303,"source_url":304,"license_code":135,"credit_html":305,"title":306,"description":195,"author":139,"original_width":307,"original_height":308},9753,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198576","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198576\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Tertiary; Ethiopia) 6.jpg",4000,3000,{"id":310,"source_url":311,"license_code":135,"credit_html":312,"title":313,"description":195,"author":139,"original_width":307,"original_height":314},9754,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198577","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198577\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Tertiary; Ethiopia) 7.jpg",2740,{"id":316,"source_url":317,"license_code":135,"credit_html":318,"title":319,"description":195,"author":139,"original_width":320,"original_height":321},9755,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198580","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198580\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Tertiary; Ethiopia) 10.jpg",1687,1136,{"id":323,"source_url":324,"license_code":135,"credit_html":325,"title":326,"description":195,"author":139,"original_width":327,"original_height":328},9757,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198587","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198587\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Tertiary; Ethiopia) 12.jpg",1768,1229,{"id":330,"source_url":331,"license_code":135,"credit_html":332,"title":333,"description":195,"author":139,"original_width":307,"original_height":334},37666,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198571","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198571\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Tertiary; Ethiopia) 5.jpg",2526,{"id":336,"source_url":337,"license_code":135,"credit_html":338,"title":339,"description":195,"author":139,"original_width":340,"original_height":341},37671,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198589","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153198589\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal (Tertiary; Ethiopia) 13.jpg",1642,1202,{"id":343,"source_url":344,"license_code":135,"credit_html":345,"title":346,"description":347,"author":139,"original_width":348,"original_height":349},60189,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=80125211","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=80125211\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Hydrophane opal (precious opal).jpg","Hydrophane opal (precious opal) dried out (Tertiary; Ethiopia)\n\u003Cp>Opal is hydrous silica (SiO2·nH2O). Technically, opal is not a mineral because it lacks a crystalline structure. Opal is supposed to be called a mineraloid. Opal is made up of extremely tiny spheres (colloids - www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence). This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids. Different opalescent colors are produced by colloids of differing sizes. If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced. Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979). \n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians. Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass. Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\nThe specimen shown above is opal-CT, which consists of extremely tiny cristobalite-tridymite aggregates called leptospheres. This is in contrast to Australian precious opal, which is opal-A (= consists of amorphous, hydrous silica colloids). The photo shows the sample dried out. Three days earlier, it was immersed in water for several hours, during which time it turned transparent. After drying out, it returned to its \"normal\" translucent state. Opal that changes its opacity in water is called hydrophane opal.",1197,1530,{"id":351,"source_url":352,"license_code":135,"credit_html":353,"title":354,"description":355,"author":139,"original_width":356,"original_height":357},60191,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500556","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500556\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Hydrophane opal (precious opal) immersed in water (Tertiary; Ethiopia) 1 (32332088570).jpg","\u003Cp>Precious opal from the Tertiary of Ethiopia. (2.7 cm across at its widest)\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids - <a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\">https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\u003C\u002Fa>\" rel=\"nofollow\">www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg<\u002Fa>) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>The specimen shown above is opal-CT, which consists of extremely tiny cristobalite-tridymite aggregates called leptospheres.  This is in contrast to Australian precious opal, which is opal-A (= consists of amorphous, hydrous silica colloids).  The photo shows the sample immersed in water - it was in water for several hours and turned from translucent to transparent.  Opal that changes its opacity in water is called hydrophane opal.\n\u003C\u002Fp>\u003Cp>Stratigraphy: precious opal nodule in rhyolitic welded tuff, Alaji Rhyolite Formation (a.k.a. Amba Alaji Rhyolite; a.k.a. Aliyu Amba Ignimbrite) OR the Shewa Robit Formation, Lower to Middle Miocene\n\u003C\u002Fp>\u003Cp>Locality: attributed to Shoa Province (Shewa Province), Ethiopia (if so, possibly from the Menz Gishe District, northeastern Shewa\u002FShoa Province, northeast-central Ethiopia)\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",1828,1423,{"id":359,"source_url":360,"license_code":135,"credit_html":361,"title":362,"description":363,"author":139,"original_width":364,"original_height":365},60192,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500558","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500558\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Hydrophane opal (precious opal) immersed in water (Tertiary; Ethiopia) 2 (32588769091).jpg","\u003Cp>Precious opal from the Tertiary of Ethiopia. (2.1 cm across at its widest)\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids - <a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\">https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\u003C\u002Fa>\" rel=\"nofollow\">www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg<\u002Fa>) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>The specimen shown above is opal-CT, which consists of extremely tiny cristobalite-tridymite aggregates called leptospheres.  This is in contrast to Australian precious opal, which is opal-A (= consists of amorphous, hydrous silica colloids).  The photo shows the sample immersed in water - it was in water for several hours and turned from translucent to transparent.  Opal that changes its opacity in water is called hydrophane opal.\n\u003C\u002Fp>\u003Cp>Stratigraphy: precious opal nodule in rhyolitic welded tuff, Alaji Rhyolite Formation (a.k.a. Amba Alaji Rhyolite; a.k.a. Aliyu Amba Ignimbrite) OR the Shewa Robit Formation, Lower to Middle Miocene\n\u003C\u002Fp>\u003Cp>Locality: attributed to Shoa Province (Shewa Province), Ethiopia (if so, possibly from the Menz Gishe District, northeastern Shewa\u002FShoa Province, northeast-central Ethiopia)\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",1308,1515,{"id":367,"source_url":368,"license_code":135,"credit_html":369,"title":370,"description":355,"author":139,"original_width":371,"original_height":372},60193,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500559","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500559\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Hydrophane opal (precious opal) immersed in water (Tertiary; Ethiopia) 3 (32332083780).jpg",1643,1184,{"id":374,"source_url":375,"license_code":135,"credit_html":376,"title":377,"description":363,"author":139,"original_width":378,"original_height":379},60194,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500560","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500560\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Hydrophane opal (precious opal) immersed in water (Tertiary; Ethiopia) 4 (32711405695).jpg",1396,1738,{"id":381,"source_url":382,"license_code":135,"credit_html":383,"title":384,"description":363,"author":139,"original_width":385,"original_height":386},60195,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500561","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500561\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Hydrophane opal (precious opal) immersed in water (Tertiary; Ethiopia) 5 (31868699304).jpg",1307,1604,{"id":388,"source_url":389,"license_code":135,"credit_html":390,"title":391,"description":392,"author":139,"original_width":393,"original_height":394},60196,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500562","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500562\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Hydrophane opal (precious opal) immersed in water (Tertiary; Ethiopia) 6 (32332077070).jpg","\u003Cp>Precious opal from the Tertiary of Ethiopia. (2.0 cm across at its widest)\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids - <a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\">https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\u003C\u002Fa>\" rel=\"nofollow\">www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg<\u002Fa>) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>The specimen shown above is opal-CT, which consists of extremely tiny cristobalite-tridymite aggregates called leptospheres.  This is in contrast to Australian precious opal, which is opal-A (= consists of amorphous, hydrous silica colloids).  The photo shows the sample immersed in water - it was in water for several hours and turned from translucent to transparent.  Opal that changes its opacity in water is called hydrophane opal.\n\u003C\u002Fp>\u003Cp>Stratigraphy: precious opal nodule in rhyolitic welded tuff, Alaji Rhyolite Formation (a.k.a. Amba Alaji Rhyolite; a.k.a. Aliyu Amba Ignimbrite) OR the Shewa Robit Formation, Lower to Middle Miocene\n\u003C\u002Fp>\u003Cp>Locality: attributed to Shoa Province (Shewa Province), Ethiopia (if so, possibly from the Menz Gishe District, northeastern Shewa\u002FShoa Province, northeast-central Ethiopia)\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",1393,1644,{"id":396,"source_url":397,"license_code":135,"credit_html":398,"title":399,"description":400,"author":139,"original_width":401,"original_height":402},60197,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500563","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500563\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Hydrophane opal (precious opal) dried out (Tertiary; Ethiopia) 1 (32671102646).jpg","\u003Cp>Precious opal from the Tertiary of Ethiopia. (2.7 cm across at its widest)\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids - <a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\">https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\u003C\u002Fa>\" rel=\"nofollow\">www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg<\u002Fa>) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>The specimen shown above is opal-CT, which consists of extremely tiny cristobalite-tridymite aggregates called leptospheres.  This is in contrast to Australian precious opal, which is opal-A (= consists of amorphous, hydrous silica colloids).  The photo shows the sample dried out.  Three days earlier, it was immersed in water for several hours, during which time it turned transparent.  After drying out, it returned to its \"normal\" translucent state.  Opal that changes its opacity in water is called hydrophane opal.\n\u003C\u002Fp>\u003Cp>Stratigraphy: precious opal nodule in rhyolitic welded tuff, Alaji Rhyolite Formation (a.k.a. Amba Alaji Rhyolite; a.k.a. Aliyu Amba Ignimbrite) OR the Shewa Robit Formation, Lower to Middle Miocene\n\u003C\u002Fp>\u003Cp>Locality: attributed to Shoa Province (Shewa Province), Ethiopia (if so, possibly from the Menz Gishe District, northeastern Shewa\u002FShoa Province, northeast-central Ethiopia)\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",1585,1336,{"id":404,"source_url":405,"license_code":135,"credit_html":406,"title":407,"description":400,"author":139,"original_width":408,"original_height":409},60198,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500569","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500569\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Hydrophane opal (precious opal) dried out (Tertiary; Ethiopia) 2 (32711478965).jpg",1681,1355,{"id":411,"source_url":412,"license_code":135,"credit_html":413,"title":414,"description":415,"author":139,"original_width":416,"original_height":394},60199,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500570","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500570\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Hydrophane opal (precious opal) dried out (Tertiary; Ethiopia) 3 (32711476645).jpg","\u003Cp>Precious opal from the Tertiary of Ethiopia. (2.4 cm across at its widest)\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids - <a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\">https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\u003C\u002Fa>\" rel=\"nofollow\">www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg<\u002Fa>) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>The specimen shown above is opal-CT, which consists of extremely tiny cristobalite-tridymite aggregates called leptospheres.  This is in contrast to Australian precious opal, which is opal-A (= consists of amorphous, hydrous silica colloids).  The photo shows the sample dried out.  Three days earlier, it was immersed in water for several hours, during which time it turned transparent.  After drying out, it returned to its \"normal\" translucent state.  Opal that changes its opacity in water is called hydrophane opal.\n\u003C\u002Fp>\u003Cp>Stratigraphy: precious opal nodule in rhyolitic welded tuff, Alaji Rhyolite Formation (a.k.a. Amba Alaji Rhyolite; a.k.a. Aliyu Amba Ignimbrite) OR the Shewa Robit Formation, Lower to Middle Miocene\n\u003C\u002Fp>\u003Cp>Locality: attributed to Shoa Province (Shewa Province), Ethiopia (if so, possibly from the Menz Gishe District, northeastern Shewa\u002FShoa Province, northeast-central Ethiopia)\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",1548,{"id":418,"source_url":419,"license_code":135,"credit_html":420,"title":421,"description":422,"author":139,"original_width":423,"original_height":424},60200,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500571","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500571\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Hydrophane opal (precious opal) dried out (Tertiary; Ethiopia) 4 (32711473695).jpg","\u003Cp>Precious opal from the Tertiary of Ethiopia. (2.0 cm across at its widest)\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids - <a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\">https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\u003C\u002Fa>\" rel=\"nofollow\">www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg<\u002Fa>) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>The specimen shown above is opal-CT, which consists of extremely tiny cristobalite-tridymite aggregates called leptospheres.  This is in contrast to Australian precious opal, which is opal-A (= consists of amorphous, hydrous silica colloids).  The photo shows the sample dried out.  Three days earlier, it was immersed in water for several hours, during which time it turned transparent.  After drying out, it returned to its \"normal\" translucent state.  Opal that changes its opacity in water is called hydrophane opal.\n\u003C\u002Fp>\u003Cp>Stratigraphy: precious opal nodule in rhyolitic welded tuff, Alaji Rhyolite Formation (a.k.a. Amba Alaji Rhyolite; a.k.a. Aliyu Amba Ignimbrite) OR the Shewa Robit Formation, Lower to Middle Miocene\n\u003C\u002Fp>\u003Cp>Locality: attributed to Shoa Province (Shewa Province), Ethiopia (if so, possibly from the Menz Gishe District, northeastern Shewa\u002FShoa Province, northeast-central Ethiopia)\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",1306,1690,{"id":426,"source_url":427,"license_code":135,"credit_html":428,"title":429,"description":400,"author":139,"original_width":430,"original_height":431},60201,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500573","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500573\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Hydrophane opal (precious opal) dried out (Tertiary; Ethiopia) 5 (32558897952).jpg",1544,1497,{"id":433,"source_url":434,"license_code":135,"credit_html":435,"title":436,"description":437,"author":139,"original_width":438,"original_height":439},60202,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500576","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500576\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Hydrophane opal (precious opal) dried out (Tertiary; Ethiopia) 7 (32711465315).jpg","\u003Cp>Precious opal from the Tertiary of Ethiopia. (2.6 cm across at its widest)\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids - <a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\">https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\u003C\u002Fa>\" rel=\"nofollow\">www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg<\u002Fa>) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>The specimen shown above is opal-CT, which consists of extremely tiny cristobalite-tridymite aggregates called leptospheres.  This is in contrast to Australian precious opal, which is opal-A (= consists of amorphous, hydrous silica colloids).  The photo shows the sample dried out.  Three days earlier, it was immersed in water for several hours, during which time it turned transparent.  After drying out, it returned to its \"normal\" translucent state.  Opal that changes its opacity in water is called hydrophane opal.\n\u003C\u002Fp>\u003Cp>Stratigraphy: precious opal nodule in rhyolitic welded tuff, Alaji Rhyolite Formation (a.k.a. Amba Alaji Rhyolite; a.k.a. Aliyu Amba Ignimbrite) OR the Shewa Robit Formation, Lower to Middle Miocene\n\u003C\u002Fp>\u003Cp>Locality: attributed to Shoa Province (Shewa Province), Ethiopia (if so, possibly from the Menz Gishe District, northeastern Shewa\u002FShoa Province, northeast-central Ethiopia)\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",1462,1176,{"id":441,"source_url":442,"license_code":135,"credit_html":443,"title":444,"description":437,"author":139,"original_width":445,"original_height":446},60203,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500577","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500577\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Hydrophane opal (precious opal) dried out (Tertiary; Ethiopia) 8 (32711463075).jpg",1628,1473,{"id":448,"source_url":449,"license_code":135,"credit_html":450,"title":451,"description":452,"author":139,"original_width":453,"original_height":454},60204,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500578","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=84500578\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Hydrophane opal (precious opal) dried out (Tertiary; Ethiopia) 9 (32558884822).jpg","\u003Cp>Precious opal from the Tertiary of Ethiopia. (2.3 centimeters across at its widest)\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids - <a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\">https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\u003C\u002Fa>\" rel=\"noreferrer nofollow\">www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg<\u002Fa>) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>The specimen shown above is opal-CT, which consists of extremely tiny cristobalite-tridymite aggregates called leptospheres.  This is in contrast to Australian precious opal, which is opal-A (= consists of amorphous, hydrous silica colloids).  The photo shows the sample dried out.  Three days earlier, it was immersed in water for several hours, during which time it turned transparent.  After drying out, it returned to its \"normal\" translucent state.  Opal that changes its opacity in water is called hydrophane opal.\n\u003C\u002Fp>\u003Cp>Stratigraphy: precious opal nodule in rhyolitic welded tuff, Alaji Rhyolite Formation (a.k.a. Amba Alaji Rhyolite; a.k.a. Aliyu Amba Ignimbrite) OR the Shewa Robit Formation, Lower to Middle Miocene\n\u003C\u002Fp>\u003Cp>Locality: attributed to Shoa Province (Shewa Province), Ethiopia (if so, possibly from the Menz Gishe District, northeastern Shewa\u002FShoa Province, northeast-central Ethiopia)\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"noreferrer nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",1418,1547,{"id":456,"source_url":457,"license_code":135,"credit_html":458,"title":459,"description":460,"author":139,"original_width":461,"original_height":462},66517,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=82968804","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=82968804\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal in basalt (Tablon Mine, near Erandique, Lempira Department, Honduras) 1 (15292029433).jpg","\u003Cp>Precious opal (\"black matrix opal\") in Tertiary-aged vesicular basalt from Honduras. (2.35 centimeters across at its widest)\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 5400 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids - <a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\">https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\u003C\u002Fa>\" rel=\"noreferrer nofollow\">www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg<\u002Fa>) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 millimeters in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 millimeters, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>Locality: Tablon Mine, near Erandique, southeastern Lempira Department, western Honduras\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"noreferrer nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",2201,1199,{"id":464,"source_url":465,"license_code":135,"credit_html":466,"title":467,"description":468,"author":139,"original_width":469,"original_height":470},66518,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=82968805","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=82968805\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal in basalt (Tablon Mine, near Erandique, Lempira Department, Honduras) 2 (15911687595).jpg","\u003Cp>Precious opal (\"black matrix opal\") in Tertiary-aged vesicular basalt from Honduras. (2.35 cm across at its widest)\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids - <a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\">https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\u003C\u002Fa>\" rel=\"nofollow\">www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg<\u002Fa>) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>Locality: Tablon Mine, near Erandique, southeastern Lempira Department, western Honduras\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",2198,1127,{"id":472,"source_url":473,"license_code":135,"credit_html":474,"title":475,"description":476,"author":139,"original_width":477,"original_height":478},66519,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=82969137","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=82969137\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal in basalt (Tablon Mine, near Erandique, Lempira Department, Honduras) 3 (27729805492).jpg","\u003Cp>Precious opal (\"black matrix opal\") in Tertiary-aged vesicular basalt from Honduras.\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are about 5400 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids - <a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\">https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\u003C\u002Fa>\" rel=\"nofollow\">www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg<\u002Fa>) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>Locality: Tablon Mine, near Erandique, southeastern Lempira Department, western Honduras\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",3508,2192,{"id":480,"source_url":481,"license_code":135,"credit_html":482,"title":483,"description":484,"author":139,"original_width":485,"original_height":486},66520,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=82969138","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=82969138\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal in basalt (Tablon Mine, near Erandique, Lempira Department, Honduras) 4 (27562762600).jpg","\u003Cp>Precious opal (\"black matrix opal\") in Tertiary-aged vesicular basalt from Honduras.\n\u003C\u002Fp>\u003Cp>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.\n\u003C\u002Fp>\u003Cp>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  \"Silica\" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens \"belong\" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.\n\u003C\u002Fp>\u003Cp>Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure.  Opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres (colloids - <a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\">https:\u002F\u002Fwww.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg\u003C\u002Fa>\" rel=\"nofollow\">www.uwgb.edu\u002Fdutchs\u002Facstalks\u002Facscolor\u002FOPALSPHR.jpg<\u002Fa>) that can be seen with a scanning electron microscope (SEM).\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced.  Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have the famous play of colors, however.  Common opal has a wax-like luster & is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians.  Some organisms incorporate opal into their tissues, for example horsetails\u002Fscouring rushes and sawgrass.  Sometimes, fossils are preserved in opal or precious opal.\n\u003C\u002Fp>\u003Cp>Locality: Tablon Mine, near Erandique, southeastern Lempira Department, western Honduras\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\n<a href=\"\u003Ca rel=\"nofollow\" class=\"external free\" href=\"http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\">http:\u002F\u002Fwww.mindat.org\u002Fgallery.php?min=3004\u003C\u002Fa>\" rel=\"nofollow\">www.mindat.org\u002Fgallery.php?min=3004<\u002Fa>\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",3582,1389,{"id":488,"source_url":489,"license_code":135,"credit_html":490,"title":491,"description":484,"author":139,"original_width":492,"original_height":493},66521,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=82969139","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=82969139\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal in basalt (Tablon Mine, near Erandique, Lempira Department, Honduras) 5 (27886419815).jpg",3423,2013,{"id":495,"source_url":496,"license_code":135,"credit_html":497,"title":498,"description":499,"author":139,"original_width":500,"original_height":501},66527,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=102415173","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=102415173\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal in basalt (Honduras) 5.jpg","Precious opal (\"black matrix opal\") in Tertiary-aged vesicular basalt from Honduras.\n\u003Cp>The tiny spots of rainbow colors in this basalt sample (click once or twice on the picture to zoom in) are composed of precious opal.  Opal is hydrous silica (SiO2·nH2O).  Technically, opal is not a mineral because it lacks a crystalline structure - opal is supposed to be called a mineraloid.  Opal is made up of extremely tiny spheres called collloids that can be only be seen with a scanning electron microscope.\n\u003C\u002Fp>\u003Cp>Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence).  This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids.  Different opalescent colors are produced by colloids of differing sizes.  If individual colloids are larger than 140 x 10 -6 mm in size, purple, blue, and green colors are produced.  Once colloids get as large as about 240 x 10 -6 mm, red color is seen (Carr et al., 1979).\n\u003C\u002Fp>\u003Cp>Not all opals have a play of colors - common opal usually has a wax-like luster and is often milky whitish with no visible color play at all.  Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.\n\u003C\u002Fp>\u003Cp>Locality: unrecorded site in Honduras (possibly - probably? - from the Tablon Mine, near Erandique, southeastern Lempira Department, western Honduras)\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Photo gallery of opal:\nwww.mindat.org\u002Fgallery.php?min=3004\n\u003C\u002Fp>\n\u003Chr>\n\u003Cp>Reference cited:\n\u003C\u002Fp>\nCarr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program.  Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51.  68 pp.",2087,1228,{"id":503,"source_url":504,"license_code":135,"credit_html":505,"title":506,"description":499,"author":139,"original_width":507,"original_height":508},66528,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=102415175","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=102415175\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal in basalt (Honduras) 4.jpg",2514,1283,{"id":510,"source_url":511,"license_code":135,"credit_html":512,"title":513,"description":499,"author":139,"original_width":514,"original_height":515},66529,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=102415176","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=102415176\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal in basalt (Honduras) 3.jpg",1167,1268,{"id":517,"source_url":518,"license_code":135,"credit_html":519,"title":520,"description":499,"author":139,"original_width":521,"original_height":522},66530,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=102415178","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=102415178\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal in basalt (Honduras) 2.jpg",969,868,{"id":524,"source_url":525,"license_code":135,"credit_html":526,"title":527,"description":499,"author":139,"original_width":528,"original_height":529},66531,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=102415181","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=102415181\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal in basalt (Honduras) 1.jpg",1557,1182,{"id":531,"source_url":532,"license_code":135,"credit_html":533,"title":534,"description":299,"author":139,"original_width":535,"original_height":536},66532,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=126617124","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=126617124\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal in basalt (Honduras) 8.jpg",1474,858,{"id":538,"source_url":539,"license_code":135,"credit_html":540,"title":541,"description":299,"author":139,"original_width":542,"original_height":543},66533,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=126617126","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=126617126\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal in basalt (Honduras) 9.jpg",1694,1476,{"id":545,"source_url":546,"license_code":135,"credit_html":547,"title":548,"description":299,"author":139,"original_width":327,"original_height":549},66534,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=126617127","James St. John, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=126617127\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","Precious opal in basalt (Honduras) 6.jpg",1609,{"id":551,"source_url":552,"license_code":282,"credit_html":553,"title":554,"description":555,"author":556,"original_width":557,"original_height":558},71562,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=7575558","Didier Descouens, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=7575558\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","OpaleMexique.jpg","Opal-AG (Precious opal)- Carbonea Mine, La Trinidad, Mun. de Tequisquiapan, Queretaro, Mexico - (5x2.2cm)","Didier Descouens",1888,2494,{"id":560,"source_url":561,"license_code":282,"credit_html":562,"title":563,"description":564,"author":565,"original_width":566,"original_height":567},71565,"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153986207","Archaeodontosaurus, via \u003Ca href=\"https:\u002F\u002Fcommons.wikimedia.org\u002F?curid=153986207\" rel=\"noopener\">Wikimedia Commons\u003C\u002Fa>","(Mexico) Opal-AG (Precious opal) Carbonea Mine Queretaro.png","Opál z Mexika","Archaeodontosaurus",3623,4869,[],[570],"Rainbow Opal",[],{"history":8,"applications":8}]