The chemical formula for gypsum is CaSO4.2H2O. Gypsum is also known as hydrous calcium sulfate. Dehydrated gypsum is colloquially known as plaster of Paris, which becomes gypsum again with the addition of water.
Gypsum is found in nature as a sedimentary evaporite mineral formed from the evaporation of salt water or as the precipitate from hot springs. Gypsum is a soft, lightweight mineral with perfect cleavage, and will split easily into flakes and plates. Gypsum is sometimes fluorescent when exposed to ultraviolet light.
The mineral can be dissolved in both hot water and hydrochloric acid. It is used to make blackboard chalk and drywall, as a flux for pottery, and as a fertilizer.