Gypsum is a soft, white or gray mineral composed of calcium sulfate dihydrate. It is commonly used in construction materials such as drywall, plaster, and cement. Selenite, Gesso, ALABASTER, Yeso, and Parget are all terms used to describe different forms of gypsum. Selenite is a transparent form of gypsum that can be found in crystal form. Gesso is a type of plaster made from gypsum and used as a primer for painting. ALABASTER is a type of gypsum that is usually white and translucent. Yeso is a Spanish term for gypsum, and Parget is a type of plaster made from gypsum and used to decorate walls.