Gypsum is a common mineral. It is found in layers that were formed under salt water millions of years ago. When water evaporated, it left the mineral behind. Gypsum is mined from sedimentary rock formations around the world. It takes the form of crystals which can at times be found projecting from rock – leading to its old English name of the Spear Stone. Gypsum is composed of calcium sulphate (CaSO4) and water (H2O). Its chemical name is calcium sulphate dihydrate (CaSO4.2H2O).
The word gypsum is derived from the Greek word γύψος (gypsos), "plaster". Because the quarries of the Montmartre district of Paris have long furnished burnt gypsum (calcined gypsum) used for various purposes, this dehydrated gypsum became known as plaster of Paris. Upon adding water, after a few tens of minutes, plaster of Paris becomes regular gypsum (dihydrate) again, causing the material to harden or "set" in ways that are useful for casting and construction.