mineral

Satin Spar (Selenite)

Gypsum (Calcium Sulfate Dihydrate, CaSO4·2H2O)

Hardness: 2 on Mohs scale (can be scratched by a fingernail). Color: Pearly white or colorless. Luster: Silky to pearly. Crystal structure: Monoclinic, typically fibrous with chatoyancy ('cat's eye' effect). Specific gravity: 2.3.

Hardness: 2 on Mohs scale (can be scratched by a fingernail)
Color: Pearly white or colorless
Luster: Silky to pearly

Identified June 6, 2026More mineral →

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Physical properties

Formation & geological history

Formed as an evaporite mineral in sedimentary environments such as saline lakes, lagoons, and caves where calcium-rich waters evaporate. Found across various geological ages from the Permian to the Holocene.

Uses & applications

Used primarily for ornamental purposes, healing crystals, and palm stones. Industrially, gypsum is used in plaster, drywall, and as a soil conditioner.

Geological facts

While often sold as 'Selenite', this fibrous variety is technically Satin Spar. True Selenite is transparent and platy. The Naica Mine in Mexico contains giant gypsum crystals that are among the largest in the world.

Field identification & locations

Identify by its extreme softness (scratches with a nail) and the silky luster of its parallel fibers. Common in Morocco, Mexico, and the USA. Collectors should keep it dry, as it is slightly water-soluble.

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