mineral

Gypsum crystal cluster (selenite/ satin-spar variety)

Gypsum — calcium sulfate dihydrate, CaSO₄·2H₂O

AI-generated identification · may be incorrect

Usually colorless to white with a vitreous-to-pearly luster and prominent sheetlike cleavage; Mohs hardness 2, monoclinic crystals, and specific gravity about 2.3. The specimen’s bladed, translucent crystals and fibrous-looking fracture strongly fit gypsum, though exact identification would require testing.

Identified August 10, 2026More mineral →

Explore Gypsum crystal cluster (selenite/ satin-spar variety) in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Forms by evaporation in sedimentary environments, especially saline lakes, playas, and evaporite basins; it can also precipitate from hydrothermal fluids. Deposits range from ancient to modern, commonly Permian through Cenozoic in age.

Uses & applications

Used to manufacture plaster, drywall, cement additives, soil conditioners, and casts; attractive transparent or well-formed clusters are sold as decorative and collecting specimens.

Geological facts

Gypsum is soft enough to scratch easily with a fingernail and can be dehydrated by heating to form plaster of Paris. Transparent massive specimens are called selenite, while fibrous material is satin spar.

Field identification & locations

Common localities include Mexico, Morocco, Spain, the United States, and Tunisia. Confirm with a fingernail scratch, perfect cleavage, low density, and lack of reaction to dilute acid; avoid confusing it with harder calcite or quartz.