Native Chromium

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.

Resistance to being scratched, ranked on the Mohs scale from 1 (talc, marked by a fingernail) to 10 (diamond, scratches everything below it). The classic field test.

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.

How a mineral resists breaking, bending, or crushing. Brittle snaps cleanly, malleable hammers into sheets, sectile slices like soft metal.

The tendency to split along flat planes that match the crystal's atomic structure. Described by quality (perfect, good, poor) and direction.

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 colour seen through a polarising microscope — often unlike the hand-specimen colour, because section thickness and orientation both matter.

The percentage of incident light reflected by a polished surface, measured at standard wavelengths. The defining property of ore microscopy.

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.

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.