Eckermannite

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.

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'.

Emission of visible light triggered by an external energy source — UV, heat, friction, or impact. Includes fluorescence and phosphorescence.

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 angles (α, β, γ) between unit-cell edges, in degrees. Cubic crystals have 90°/90°/90°; triclinic crystals can take any combination.

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

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.

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