mineral

Fibrous gypsum (selenite/satin-spar variety)

Gypsum, CaSO₄·2H₂O

AI-generated identification · may be incorrect

Likely fibrous, earthy-stained gypsum with silky to dull luster and pale brown color. Mohs hardness 2; monoclinic crystals, perfect cleavage, and relatively low specific gravity (~2.3).

Identified October 13, 2026More mineral →

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Physical properties

Likely fibrous, earthy-stained gypsum with silky to dull luster and pale brown color. Mohs hardness 2; monoclinic crystals, perfect cleavage, and relatively low specific gravity (~2.3).

Formation & geological history

Forms when calcium-sulfate-rich waters evaporate in sedimentary basins, or through hydration of anhydrite; deposits range from recent to hundreds of millions of years old. The image alone cannot establish the specimen’s age or locality.

Uses & applications

Gypsum is used in drywall, plaster, cement, and soil amendment; attractive fibrous material is occasionally cut or collected as a specimen.

Geological facts

Gypsum is soft enough to be scratched by a fingernail and can be scratched with a copper coin. Its fibrous variety is commonly called satin spar; the photo’s rusty-brown color may reflect iron-oxide staining.

Field identification & locations

Look for fibrous or tabular crystals, very low hardness, and a white streak; avoid identifying solely by color. Gypsum occurs worldwide in evaporite deposits, including major beds in the United States, Mexico, and Spain.