A naturally occurring crystalline form of hydrated calcium sulfate. Gypsum is a calcium dihydrate (CaSO<sub>4</sub> • 2H<sub>2</sub>O) in the monoclinic crystalline form. At elevated temperatures the dihydrate in a dry environment may lose some of its water of crystallization to become the unstable hemihydrate (which will rehydrate when exposed to water). A monoclinic crystal of gypsum cannot become an orthorhombic crystal of anhydrite by simple [[dehydrate|dehydration]]. Compare [[anhydrite]].