mineral

Selenite (gypsum crystal)

Gypsum, calcium sulfate dihydrate (CaSO₄·2H₂O), likely the variety selenite

AI-generated identification · may be incorrect

Colorless to white, translucent, pearly-to-glassy luster, and typically monoclinic crystal structure with excellent cleavage; Mohs hardness 2, specific gravity about 2.3. The fibrous vertical striations and soft, easily scratched surface are characteristic.

Identified August 11, 2026More mineral →

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Physical properties

Formation & geological history

Forms by evaporation of saline or alkaline water in sedimentary basins, caves, and hydrothermal settings; deposits range from recent to hundreds of millions of years old. This specimen appears to be a natural cleavage fragment or partially shaped crystal.

Uses & applications

Used extensively to manufacture plaster, drywall, cement, and soil amendments; clear specimens are collected and sometimes used decoratively, but are too soft for most jewelry.

Geological facts

Selenite is not the same as the fibrous mineral variety satin spar, though both are gypsum. It can be scratched with a fingernail and should not be soaked because gypsum is slightly water-soluble.

Field identification & locations

Identify by its very low hardness, perfect cleavage, white translucency, and fibrous striations; common localities include Mexico, Morocco, Spain, the United States, and Australia. Avoid abrasion, moisture, and heat.