Gypsum forms by the process of evaporation of seawater, which leads to the precipitation of various salts in evaporite deposits such as gypsum. Saccharoidal, the most important commercially, resembles lump sugar and occurs as massive beds.
Satin spar is fibrous in appearance, has a satiny luster, and forms thin discontinuous layers.
Selenite is a transparent, often colorless, crystalline variety. Small amounts of very pure gypsum are used as fillers, in glass making, and in pharmaceutical applications. Arkansas gypsum is used primarily in the manufacture of wallboard and as a cement additive. When added to cement, gypsum slows the setting time or crystallization of cement in concrete, allowing mixed concrete to be trucked longer distances. However, gypsum is rarely used as an industrial source because sulfur is generally available from non-gypsum resources at a lower cost.