Downeyite

The kinds of rock environments where the mineral typically forms — igneous, metamorphic, hydrothermal, sedimentary, pegmatitic, and so on.

The place where the mineral was first found and described — by convention, the reference point for the species.

How the mineral's surface reflects light — from metallic and glassy to silky, pearly, or dull. Often the first clue to identification in the field.

Whether and how much light passes through a mineral, from transparent (you can see through it) to translucent, then opaque.

The hue of the mineral itself. Often unreliable for identification — many species occur in several colours depending on trace impurities or radiation damage.

The colour of a mineral's powder, drawn by scraping it across an unglazed porcelain plate. Far more consistent than surface colour.

Mass per unit volume, in grams per cubic centimetre. A useful sorting tool: barite is twice as dense as quartz at the same size.

Whether the mineral is isotropic (light travels at one speed in all directions, like in glass) or anisotropic — uniaxial or biaxial. Defines which optical tests apply.

The difference between the highest and lowest refractive indices. Drives the interference colours seen under crossed polarisers.

The orientation, relative to crystal edges, at which a thin section goes dark between crossed polarisers. Used to separate look-alike species.

How the mineral glows under short- and long-wave ultraviolet lamps. Some species fluoresce in spectacular colours invisible to the naked eye.

One of seven geometric families that describe a mineral's atomic symmetry: isometric (cubic), tetragonal, hexagonal, trigonal, orthorhombic, monoclinic, triclinic. Two non-crystalline classes — amorphous and icosahedral — also appear.

The full symmetry description of a crystal's repeating atomic pattern — 230 distinct groups, written in Hermann–Mauguin notation (e.g. P6₃/mmc).

The lengths of the unit-cell edges (a, b, c) in ångströms — the smallest repeating box of atoms that builds the crystal.

The number of formula units contained in a single unit cell.

The overall shape and habit a mineral tends to grow into — cubes, prisms, plates, fibres, druses, masses.

Two or more crystals of the same species grown in a fixed symmetric relationship, sharing a plane or axis. Often visible as re-entrant angles.

The crystal habit observed at the type locality — the place where the mineral was first described.

Chemical elements that make up the mineral's ideal formula, listed by atomic symbol with their share of the total mass.