Gypsum is the most common of the naturally occurring sulfate minerals. It is found in very extensive, bedded sedimentary deposits all over the world and is associated with limestones, shales and sandstones, marls and clays. High-purity deposits of gypsum are relatively common, and, for most uses, little or no beneficiation is required. In the United States most gypsum rock is finely ground and partially calcined to drive off 75 pct of the chemically combined water, converting gypsum, caso4.2H2o, to the hemihydrate product, caso4.1/2H2o. Commonly called plaster of paris, this material quickly sets and hardens back to gypsum when mixed with the appropriate amount of water.