what is gypsum used in
Gypsum crystals in the Cave of the Crystals in Mexico. Note person for scale. Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO 4 Â·2H 2 O. It can be used as a fertilizer, is the main constituent in many forms of plaster and in blackboard chalk, and is widely mined.he word gypsum is derived from the Greek word Î³ÏÏÎ¿Ï (gypsos), chalk or 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.