Gypsum, a soft, white mineral consisting of calcium, sulfur, and water. Gypsum is found in thick beds deposited by ancient seas. It is mined all over the world. Gypsum occurs in five forms:

Rock Gypsum

the common form, is scaly or granular. The plasterboard used by builders is made of rock gypsum, pressed into layers with paper and felt. Rock gypsum is also used in portland cement and in the manufacture of plate glass and water-base paints. It is calcined (heated to drive off water) to make plaster of Paris.

Gypsite

an impure, earthy form, is used as a fertilizer, particularly for peanut crops in the southern United States.

Alabaster

is a compact form, easily carved into statues and lamp bases.

Selenite

is the transparent crystal form, used for inexpensive jewelry.

Satin Spar

is fiberlike, with a silky luster. It has some ornamental uses.

Gypsum is hydrous calcium sulfate. Its formula is CaSO42H2O.