Ferro-ferri-holmquistite

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.

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.

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.

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.

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

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.

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

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.