Gypsum may act as a source of sulfur for plant growth, which was discovered by J. Mayer, and in the early 19th century, it was regarded as an almost miraculous fertilizer.American farmers were so anxious to acquire it that a lively smuggling trade with Nova Scotia evolved, resulting in the so-called "Plaster War" of 1820.
As for anhydrite, its solubility in saline solutions and in brines is also strongly dependent on Na Cl concentration.
Gypsum occurs in nature as flattened and often twinned crystals, and transparent, cleavable masses called selenite.
Selenite contains no significant selenium; rather, both substances were named for the ancient Greek word for the Moon.
It is widely mined and is used as a fertilizer, and as the main constituent in many forms of plaster, blackboard chalk and wallboard.
A massive fine-grained white or lightly tinted variety of gypsum, called alabaster, has been used for sculpture by many cultures including Ancient Egypt, Mesopotamia, Ancient Rome, Byzantine empire and the Nottingham alabasters of medieval England.
It is the definition of a hardness of 2 on the Mohs scale of mineral hardness.
It forms as an evaporite mineral and as a hydration product of anhydrite.
Because the quarries of the Montmartre district of Paris have long furnished burnt gypsum (calcined gypsum) used for various purposes, this dehydrated gypsum became known as plaster of Paris.
Upon addition of water, after a few tens of minutes plaster of Paris becomes regular gypsum (dihydrate) again, causing the material to harden or "set" in ways that are useful for casting and construction.
Gypsum was known in Old English as spærstān, "spear stone", referring to its crystalline projections.
(Thus, the word spar in mineralogy is by way of comparison to gypsum, referring to any non-ore mineral or crystal that forms in spearlike projections).