Our glossary for a range of mineralogical terms – this will be added to over time as I think of others, or come across words I don’t know!
Amorphous – Lacking a definite crystal structure (e.g., opal, obsidian).
Anhedral – A mineral grain without well-formed crystal faces.
Asbestiform – A fibrous, flexible crystal habit (e.g., chrysotile).
Birefringence – The optical property causing double refraction in anisotropic minerals.
Botryoidal – A grape-like mineral surface texture (e.g., hematite, malachite).
Bravais Lattices – The 14 unique 3D lattice systems that crystals are based on.
Cleavage – The tendency of a mineral to break along specific planar surfaces.
Conchoidal Fracture – Curved breakage pattern typical of quartz or glass.
Crystal Habit – The external shape a crystal tends to form.
Crystal System – Classification based on symmetry (cubic, hexagonal, monoclinic, etc.).
Density – Mass per unit volume, used to help identify minerals.
Dendritic – A branching, tree-like crystal habit.
Dichroism – Color change when viewed from different angles in polarized light.
Effervescence – Reaction of a mineral with acid (e.g., calcite fizzing with HCl).
Elemental Mineral – A mineral composed of a single element (e.g., gold, copper).
Euhedral – Well-formed crystal faces; the opposite of anhedral.
Exsolution – Separation of different minerals from a solid solution during cooling.
Feldspar – A common group of rock-forming silicate minerals.
Ferroan – Containing iron (Fe), as in ferroan dolomite.
Fluorescence – Emission of light by a mineral under UV light.
Fracture – The pattern in which a mineral breaks other than along cleavage planes.
Gemmology – The study of gemstones, often a subfield of mineralogy.
Habit – The common or characteristic shape of a mineral crystal.
Hardness – A measure of scratch resistance, often determined with the Mohs scale.
Idiochromatic – A mineral colored by its essential chemical components.
Inclusions – Impurities or other materials trapped within a mineral crystal.
Isomorphous – Minerals with the same crystal structure but different chemical compositions.
Luster – The way a mineral reflects light (metallic, vitreous, dull, etc.).
Mafic – Silicate minerals rich in magnesium and iron (e.g., olivine, pyroxene).
Massive – A mineral occurrence without visible crystal shapes.
Micaceous – Having a flaky or layered habit, like mica.
Mineraloid – A substance that resembles a mineral but lacks crystallinity (e.g., obsidian).
Mohs Scale – A scale ranking minerals by hardness from talc (1) to diamond (10).
Nesosilicate – Silicate minerals with isolated SiO₄ tetrahedra (e.g., olivine, garnet).
Optical Axis – The direction in a crystal along which light experiences no birefringence.
Ore Mineral – A mineral that contains economically valuable metal(s).
Pseudomorph – A mineral that has replaced another mineral’s structure but retains its shape.
Pleochroism – The appearance of different colors from different angles in polarized light.
Polymorph – Minerals with the same chemical composition but different structures (e.g., graphite and diamond).
Pyroelectricity – Temporary electric charge developed in a crystal due to temperature change.
Refractive Index – A measure of how light bends through a mineral.
Silicate – The most abundant class of minerals, built from silicon-oxygen tetrahedra.
Sphalerite – A common zinc ore mineral and example of high dispersion and cleavage.
Streak – The color of a mineral’s powder when rubbed on a porcelain plate.
Striations – Fine parallel grooves or lines on a crystal face.
Subhedral – Partially developed crystal faces.
Sulfide – A group of minerals composed of sulfur and a metal (e.g., pyrite).
Twinning – Symmetrical intergrowth of two or more crystal segments.
Vitreous – Glassy luster, common in quartz and feldspar.
Zoning – Variations in composition within a single crystal, often seen in plagioclase or garnet.