Gypsum is hydrous calcium sulphate having chemical composition CaSO4.2H2O (cf.,
composition of anhydrite is CaSO4). It has five varieties, namely:

  1. Alabaster: The massive fine grained granular compact type; the name is related to a
    place in ancient Egypt called Alabaston, where this mineral used to be quarried for
    sculpturing (it is not known for certain whether the name of the place was after the
    name of the mineral or the other way around).
  2. Selenite: The transparent and crystalline type, name originating from the Greek word
    “selen” meaning moon (cf., name of the metalloid selenium from the Greek word
    “selas” meaning light).
  3. Satinspar: The fibrous type with silky lustre; the name coming from the silky fabric
    called satin (it is believed that this fabric used to be exported from the ancient
    Chinese city named “Zaitun” present Tsinkiang from which the name satin was
  4. Gypsite (also called gypseous clay): The porous earthy impure gypsum mixed with
    sand and clay.
  5. Rock gypsum: Medium to coarse grained gypsum forming extensive thick
    sedimentary beds.
    Out of these, the first two, rock gypsum and alabaster, are of the most significant from an
    economic point of view and are most widely mined and used. Usually, people do not
    differentiate between these two and refer to them by the generalized term gypsum. But all the
    types are chemically alike and exhibit the same properties.
    Gypsum is a very common mineral found in nature (very frequently, in association with
    anhydrite). Most of the gypsum deposits originated due to evaporation of salt-rich waters
    seawater or lake brine or spring water or underground saline water rising to the surface by
    capillary force. Some deposits also originated due to hydration of anhydrite (CaSO4) and a
    few vein deposits, due to action of sulphuric acid (coming along with volcanic eruptions or
    forming by oxidation of sulphide minerals) on limestone.