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Smithsonian National Museum of Natural History
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Gypsum

Photograph of clear gypsum crystals (148357) from the National Mineral Collection
Photo by Chip Clark. Click to zoom.
Catalog Number NMNH 148357-00
Locality Chihuahua, Mexico

Gypsum is hydrated calcium sulfate and a relatively common mineral. It is used in construction as drywall and an ingredient in plaster and fertilizer. The name is derived from the Greek “gypsos” meaning “chalk or plaster.” Gypsum is typically colorless to white, but due to impurities it can occur in shades of yellow, blue, pink, brown or gray. It typically forms as flat, elongated prismatic crystals that are mostly transparent and often times twinned. This cluster of gypsum has clear spear-like or bladed crystals and grew in a cavity in limestone. Gypsum from Chihuahua, Mexico can occur as truly spectacular giant crystals forming in large cavities or voids, even in caves where they can reach sizes up to 50 feet in length and 4 feet in diameter.

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