mineral

Gypsum (selenite)

Gypsum, CaSO₄·2H₂O; crystalline variety commonly called selenite

AI-generated identification · may be incorrect

Colorless to silvery-white with a pearly or vitreous luster and prominent cleavage; monoclinic crystals. Mohs hardness 2, specific gravity about 2.3; the soft surface and sheet-like cleavage fit the specimen shown.

Identified October 14, 2026More mineral →

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Physical properties

Formation & geological history

Forms when calcium-sulfate-rich water evaporates in sedimentary basins, and in veins or caves; deposits range from ancient to geologically recent. It is an evaporite mineral, not an igneous or metamorphic rock.

Uses & applications

Used to make plaster, drywall, and cement; clear, attractive selenite is also collected and used decoratively, though it is too soft for most jewelry.

Geological facts

Gypsum can split into thin, flexible sheets and is soft enough to scratch with a fingernail. Its fibrous variety is called satin spar, while transparent crystals are often sold as selenite.

Field identification & locations

Look for very soft, pale material with smooth cleavage faces and a pearly sheen; common in evaporite deposits worldwide, including the United States, Mexico, and Spain. Avoid wetting or abrasion, which can damage it.