mineral

Selenite (gypsum)

Gypsum, CaSO₄·2H₂O; fibrous variety often called satin spar

AI-generated identification · may be incorrect

Typically Mohs hardness 2, white to translucent, vitreous-to-silky luster, monoclinic crystals, and perfect cleavage in one direction. The parallel fibers and pearly translucence strongly suggest satin-spar gypsum rather than glass.

Identified September 4, 2026More mineral →

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Physical properties

Formation & geological history

Forms when evaporating saline waters precipitate calcium sulfate, commonly in sedimentary evaporite beds; many specimens are geologically young, though gypsum deposits range from Paleozoic to modern. Common localities include Mexico, Morocco, Spain, the United States, and desert basins worldwide.

Uses & applications

Used to manufacture plaster, drywall, cement additives, and fertilizer; attractive satin-spar pieces are sold as decorative specimens and metaphysical objects, but they are too soft for most jewelry.

Geological facts

Gypsum contains chemically bound water and can be scratched easily with a fingernail. It may fluoresce under ultraviolet light, and its fibrous variety produces a cat’s-eye-like sheen when polished.

Field identification & locations

Identify it by its low hardness, light weight, perfect cleavage, and silky parallel fibers; a fingernail should scratch it, while it should not scratch glass. Avoid soaking or abrasion, since gypsum is water-soluble and easily damaged.