Gypsum

A soft, water-bearing calcium sulfate formed as seawater and lakes evaporate. Clear crystals are called selenite; sand-filled rosettes are 'desert roses'.

How to identify gypsum

Scratches with a fingernail (hardness 2).
Clear blades that bend slightly before snapping.
Doesn't fizz in acid.
Feels warm, not cold like glass.

Often confused with

Calcite
Calcite is too hard for a fingernail and fizzes in acid.
Compare side by side
Muscovite
Mica peels into elastic sheets that spring back; gypsum flakes stay bent.
Compare side by side

Where it's found

Naica, Mexico
White Sands, New Mexico
Oklahoma

Uses

Drywall, plaster of Paris and fertilizer.

Rock tumbling

Not suitable for tumbling.

Did you know?

Mexico's Cave of the Crystals at Naica holds gypsum crystals up to 12 meters long.

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