Andradite

Andradite carries the name of a Brazilian statesman who never knew the mineral would be his namesake. He died in 1838; the species took his name thirty years later.

The story begins in 1800, in the iron country of southern Norway. A Brazilian mineralogist was examining a yellowish-grey mineral from a mine near Drammen. His name was José Bonifácio de Andrada e Silva, born in Santos in 1763. He held the sample in the flame of a blowpipe — the brass tube mineralogists used to direct a candle flame onto a specimen. The substance changed colour as it cooled. Andrada named it allochroite, from the Greek allos — another — and chroia — colour.

Demantoid and the Russian gem trade

While allochroite sat quietly in the literature, prospectors in the central Urals stumbled on the variety that would make the species famous. In 1851, vivid green crystals were recovered from an alluvial deposit about 110 kilometres north-northwest of Ekaterinburg, along the Bobrovka River. The stones combined the brilliance of diamond with a saturated grass-green colour, and they were named demantoid — diamond-like — for the resemblance. Russian demantoid carries a signature no later deposit has reproduced. Inside the stone, golden feathery fibres of chrysotile — a serpentine-group asbestos mineral — radiate from a tiny central crystal of chromite. Chromite is the chromium-iron oxide that gives the variety its colour. Cutters call the inclusions horsetails.

Andradite gets its modern name

The reclassification came in 1868. The American mineralogist James Dwight Dana was then assembling the System of Mineralogy — the comprehensive treatise that would standardise much of the field. He regrouped the calcium-iron garnets under a single name and honoured Andrada's early work by calling the species andradite. Andrada's allochroite became a subvariety of the new name.

From the 1880s through the Russian Revolution, demantoid moved through the workshops of Peter Carl Fabergé and the jewellers of imperial Saint Petersburg. The trade collapsed with the empire that supported it. Demantoid all but disappeared from the market until late in the 20th century, when two finds reopened the supply. A deposit at the Green Dragon mine in Namibia was located in 1996, and a second, larger find followed in Madagascar around 2009.

Two further varieties carry older names. Melanite — from the Greek melas, black — is the opaque, titanium-bearing form. Limited substitution of titanium for iron darkens the crystal to coal-black. Topazolite is the yellow to greenish-yellow variety, rarer than demantoid and occasionally cut into faceted gems. A historical varietal name, colophonite, was given to coarse, resin-coloured masses from Scandinavian skarns.

Andradite's working life today is almost entirely a gem story. The industrial garnet trade — sandblasting grit, waterjet powder, coated abrasive paper — runs on almandine, the harder, iron-rich garnet of the same family. Andradite has no significant share of that market. Its place is on a jeweller's bench and in a collector's cabinet.

The defining gem variety is demantoid — the chromium-bearing, vivid-green form. Its name points to its diamond-like dispersion: the optical property that makes a faceted stone throw rainbow flashes of light. Demantoid is among the most prized of all garnet varieties. Russian material is sought specifically for its horsetail inclusions of chrysotile fibres. Here a flaw is part of the value — the horsetails identify the Ural origin. Younger sources in Namibia and Madagascar supply most of the contemporary trade. Smaller deposits in Iran's Kerman and West Azerbaijan provinces add to the market.

Topazolite, the yellow to greenish-yellow variety, is occasionally cut into faceted gems but is rarer than demantoid and rarely seen at the counter. Melanite, the opaque black titanium-bearing variety, is cut and polished mainly as a collector and ornamental stone rather than as a mainstream jewel.

Beyond the gem trade, well-crystallised specimens are sought by museums and private collectors as representative material of the calcium-iron garnet end-member. Demand concentrates on the classic localities: Drammen, the Bobrovka placers of the Urals, and the Green Dragon mine in Namibia.