Gypsum is found in nature in mineral and rock form. As a mineral, it can form very pretty, and sometimes extremely large, crystals. As a rock, gypsum is a sedimentary rock, typically found in thick beds or layers. It forms in lagoons where ocean waters high in calcium and sulfate content can slowly evaporate and be regularly replenished with new sources of water.
The result is the accumulation of large beds of sedimentary gypsum. Because it is deposited in this environment, it is common for gypsum to be associated with rock salt and sulfur deposits.
The mineral name gypsum is so old that it is not known who originated its use. It was derived from the Greek wordgypsos which means plaster. Originally it referred to the form of gypsum which has been heated to a high temperature to drive off the water in its crystal structure; this is called calcined gypsum. This is called Plaster of Paris.