Fluorite

A colorful mineral that grows as cubes and splits into octahedrons. Many specimens glow under ultraviolet light — the word 'fluorescence' was coined from it.

How to identify fluorite

Cubic crystals, often with color zoning in purple and green.
Cleaves into eight-sided octahedral fragments.
Hardness 4 — a knife scratches it, but a coin doesn't.
Often glows blue or violet under a UV flashlight.

Often confused with

Amethyst
Amethyst is harder (7), forms pointed six-sided crystals and has no cleavage.
Compare side by side
Calcite
Calcite fizzes in acid and cleaves into rhombs, not octahedrons.
Compare side by side

Where it's found

Cave-in-Rock, Illinois
Weardale, England
Hunan, China
Coahuila, Mexico

Uses

Flux in steelmaking, hydrofluoric acid production and low-dispersion camera lenses.

Rock tumbling

Can be tumbled with care. Cleaves easily; tumble on its own and expect chips.

Did you know?

Fluorite is the reference mineral for hardness 4 on the Mohs scale.

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