Armalcolite

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.

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.

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.

A change of colour as a coloured anisotropic mineral is rotated under plane-polarised light. Tourmaline and cordierite are textbook examples.

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

Whether an opaque mineral changes brightness or hue as the stage is rotated under crossed polarisers in reflected light. A key ore-mineral test.

A general label for how an opaque mineral responds to polarised reflected light, summarising bireflectance and anisotropism together.

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

Edge-length ratios of the unit cell, normalised so b = 1. Lets you compare related species without quoting absolute ångström values.

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

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.