- published: 25 Jul 2013
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Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs scale of mineral hardness. It forms as an evaporite mineral and as a hydration product of anhydrite.
The word gypsum is derived from the Greek word γύψος (gypsos), "chalk" or "plaster". Because the gypsum from the quarries of the Montmartre district of Paris have long furnished burnt gypsum used for various purposes, this material has been called plaster of Paris.
Gypsum was known in Old English as spærstān, "spear stone", referring to its crystalline projections. (Thus, the word spar in mineralogy is by way of comparison to gypsum, referring to any nonore mineral or crystal that forms in spearlike projections.) In the early 19th century, it was regarded as an almost miraculous fertilizer. American farmers were so anxious to acquire it, a lively smuggling trade with Nova Scotia evolved, resulting in the so-called "Plaster War" of 1812.