mineral
Polished selenite (satin spar gypsum), likely carved/tumbled
Gypsum, CaSO₄·2H₂O; varietal identification is uncertain from the image
AI-generated identification · may be incorrectWhite to translucent, pearly/satiny luster, Mohs hardness 2, very low specific gravity (~2.3), and perfect cleavage in one direction. The radial fan-like internal pattern and smooth rounded surface are consistent with polished gypsum, though white calcite is a possible alternative.
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Physical properties
White to translucent, pearly/satiny luster, Mohs hardness 2, very low specific gravity (~2.3), and perfect cleavage in one direction. The radial fan-like internal pattern and smooth rounded surface are consistent with polished gypsum, though white calcite is a possible alternative.
Formation & geological history
Gypsum forms when sulfate-rich water evaporates in sedimentary basins, commonly during Permian to Cenozoic evaporite deposition. Selenite crystals can also form in caves and clay-rich lake deposits.
Uses & applications
Used for plaster, drywall, cement, fertilizer, and decorative specimens; carved pieces are sold as inexpensive collectibles. It is generally unsuitable for durable jewelry because it scratches and dissolves relatively easily.
Geological facts
Gypsum is one of the softest common minerals and can be scratched with a fingernail. A knife or copper coin can mark it, while dilute vinegar normally produces no strong fizz.
Field identification & locations
Confirm with a fingernail-scratch test and compare with calcite: calcite fizzes in dilute hydrochloric acid, whereas gypsum does not. Common sources include Mexico, Morocco, Spain, the United States, and Australia.
