Gypsum | Encyclopedia.com
gypsum (jĬp´səm), mineral composed of calcium sulfate (calcium, sulfur, and oxygen) with two molecules of water, CaSO 4 ·2H 2 O. It is the most common sulfate mineral, occurring in many places in a variety of forms. A transparent crystalline variety is selenite. A massive gypsum of delicate color and texture, readily worked into ornamental vases, boxes, and the like, is called alabaster.