According to Georgia Southwestern State University, rock gypsum is formed by the evaporation of concentrated solutions like seawater. It is a chemical precipitate with a texture ranging from massive or granular to fibrous, depending on the shape and size of the gypsum crystals.
The common sedimentary environments where rock gypsum occurs are desert lakes and marine regions with high evaporation rates, states Georgia Southwestern State University.
In the educational material provided by Michigan State University, it is stated that gypsum is a non-metallic mineral that occurs in rock form that consists of calcium sulphate and water. Gypsum appears white in absolutely pure form. However, it normally contains impurities that make the rock look brown, gray, pink or even black. Gypsum deposits are found interlayered with limestone or shale in flat beds. Gypsum deposits formed millions of years ago when the salt water oceans receded and caused the formation of inland "dead" seas. As water evaporated, the seas became more salty and when the salts precipitated, they created many compounds including gypsum. After millions of years, the salt deposits mixed with minerals and decayed vegetation, eventually resulting in stratified rock which contained alternating layers of gypsum and limestone. The entire layers were covered with thick glacial deposits.