Montanite

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

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.

How much light bends as it enters the mineral. Higher index means stronger bending — diamond's extreme index is why it sparkles.

How sharply a grain stands out against the mounting medium (Canada balsam, n ≈ 1.54) under the petrographic microscope. Low relief means the grain almost dissolves into the slide; very high relief gives crisp, almost three-dimensional outlines.

The refractive indices measured along the crystal's principal optical axes — nω and nε for uniaxial minerals, nα, nβ, and nγ for biaxial.

How strongly the refractive indices vary across the visible spectrum. Strong dispersion gives gemstones their 'fire'.

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.

Volume of the unit cell in cubic ångströms (ų). Reflects how tightly the atoms are packed for a given chemistry.

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.