Upon adding water, after a few dozen 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.

Thus, the word spar in mineralogy, by comparison to gypsum, refers to any non-ore mineral or crystal that forms in spearlike projections.

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 with anhydrite, the solubility of gypsum in saline solutions and in brines is also strongly dependent on sodium chloride (common table salt) concentration.

A very fine-grained white or lightly tinted variety of gypsum, called alabaster, is prized for ornamental work of various sorts.

In arid areas, gypsum can occur in a flower-like form, typically opaque, with embedded sand grains called desert rose.

[16] Gypsum is deposited from lake and sea water, as well as in hot springs, from volcanic vapors, and sulfate solutions in veins.

[17] Pure gypsum is white, but other substances found as impurities may give a wide range of colors to local deposits.

This can lead to accumulation of elemental sulfur in oil-bearing formations,[19] such as salt domes,[20] where it can be mined using the Frasch process[21] Electric power stations burning coal with flue gas desulfurization produce large quantities of gypsum as a byproduct from the scrubbers.

Large open pit quarries are located in many places including Fort Dodge, Iowa, which sits on one of the largest deposits of gypsum in the world,[26] and Plaster City, California, United States, and East Kutai, Kalimantan, Indonesia.

Several small mines also exist in places such as Kalannie in Western Australia, where gypsum is sold to private buyers for additions of calcium and sulfur as well as reduction of aluminum toxicities on soil for agricultural purposes.

This product is pure enough to replace natural gypsum in a wide variety of fields including drywalls, water treatment, and cement set retarder.

As a result, waste gypsum is stored in stacks indefinitely, with significant risk of leaching their contaminants into water and soil.

Calcium sulfate per se is nontoxic and is even approved as a food additive,[34] but as powdered gypsum, it can irritate skin and mucous membranes.