mineral
Gypsum (selenite variety)
Gypsum — calcium sulfate dihydrate, CaSO₄·2H₂O
AI-generated identification · may be incorrectUsually colorless to white with a pearly or glassy luster; hardness 2 Mohs, very soft and easily scratched by a fingernail. It has excellent cleavage, commonly forms thin transparent sheets, and has a specific gravity near 2.3; the pictured piece appears cleaved or lightly shaped.
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Physical properties
Usually colorless to white with a pearly or glassy luster; hardness 2 Mohs, very soft and easily scratched by a fingernail. It has excellent cleavage, commonly forms thin transparent sheets, and has a specific gravity near 2.3; the pictured piece appears cleaved or lightly shaped.
Formation & geological history
Gypsum forms when saline water evaporates in sedimentary evaporite basins, often alongside halite and anhydrite. Deposits range from ancient Permian and Jurassic beds to modern salt lakes and caves.
Uses & applications
Used extensively for drywall, plaster, cement additives, soil amendment, and casts; clear selenite is sold as a decorative or metaphysical specimen. It is generally inexpensive.
Geological facts
Gypsum is hydrated calcium sulfate and can lose water when heated, converting toward anhydrite or plaster. The name selenite refers to transparent, sheet-form gypsum rather than a separate mineral.
Field identification & locations
Identify it by its extreme softness, layered cleavage, white translucency, and ability to be scratched with a fingernail; unlike glass, it is not hard and commonly flakes along cleavage planes. Common localities include Mexico, Morocco, Spain, the United States, and Brazil.
