Gypsum is a non-metallic mineral, found in rock form. It is composed of 79.1% calcium sulphate and 20.9% water, by weight. Chemists call it Hydrous Calcium Sulphate, and as there is one molecule of calcium sulphate combined with two molecules of water. It has the chemical formula CaSO 4 2H 2 0. By volume this works out to nearly 50% water in