Gypsum is a mineral with a hardness of 2 out of 10 on the Mohs scale of mineral hardness [?]. These Monoclinicly structured gems are made of hydrated calcium sulfate, their full chemical compound being CaSO42H2O.
Gypsum is the most common sulphate mineral. It is usually the first evaporite mineral to be precipitated form water due to its poor solubility. Varieties include Selenite (or "spectacle stone), which is colorless and transparent; Satin Spar, the fibrous, translucent form with silky luster, which when cut cabochon shows pearly chatoyant effects; Alabaster, used for ornaments, which is firm, fine-grained, massive, granular, porous, and takes color easily; and Helictite, a grotesque shaped variety, usually resembling a stalagmite.
It appears in clear, tabular crystals up to 1 meter long, often in swallowtail or spearhead twins, also as transparent crystals and cleavage fragments (selenite); fibrous aggregates of elongated, satiny crystals (satin spar); granular and compact, waxy-looking masses, sometimes banded (alabaster); also occurs in rosette-shaped aggregates, often incorporating grains of sand, of a reddish color, known as "desert roses." Radiating forms are called "daisy gypsum." These are formed in desert areas by the evaporation of rising groundwater.
Its color is white, gray, yellowish or brown.
It is soft and light, has perfect cleavage into slightly flexible, but inelastic plates and very fine flakes. It is transparent, with vitreous or silky luster, often pearly on cleavage faces. It is sometimes fluorescent in ultraviolet light, and is soluble in hydrochloric acid and hot water. It fuses fairly easily, turning cloud and opaque in the flame, with loss of water.
Gypsum is a typical sedimentary evaporite mineral, and forms through direct precipitationfrom saline waters that have no river outlets, or through alteration of anhydrite. It may also form by direct sublimation from fumaroles or precipitate from hot volcanic springs.
In England some specimens of gypsum minerals are found in thick nodular beds, which are called 'floors,' or lenticular masses known as 'cakes.'
Giant crystals are found in clay near Bologna and Pavia in Italy and in the sulfur mines of Sicily, at Chihuahua (Mexico), in Chile and in Utah (USA). Splendid "desert roses" occur in Tunisia, Morocco, and Arizona and New Mexico (USA). Stratified deposits are worked intensively in the Paris basin (France), in Nova Scotia (Canada), at Volterra (Pisa, Italy) and in many parts of the USA, in the western Urals, northern Caucasus, Uzbekistan.
The pink color alabaster from South Wales is named as pink Welsh alabaster. Also spelled gyp, gypsite, plaster stone, gyps, or plaster of Paris. Even though massive gypsum occurs worldwide, only in few localities are quarries worked for ornamental alabaster. One of the important localities for alabaster is the quarries situated at Castellina, in the district of Volterra in Tuscany, Italy. The alabaster from this district is found in nodular masses embedded in limestone interstratified with marls.
Gypsum derives its name from the Greek "gypsos" meaning "chalk" or "plaster."
Its mineral form, alabaster, has been used for sculpture in Ancient Egypt and Mesopotamia.
When water is added to dehydrated gypsum (calcined or burnt gypsum from the quarries of Montmartre, Paris) what became known as "plaster of Paris" becomes regular gypsum (dihydrate) again, and the material hardens in ways that are useful for casting and construction.
Gypsum was known in Old English as "spear stone", referring to its crystalline projections. It was also source of sulfur for plant growth, and was a much sought-after fertilizer in the early 19th century.
Gypsum is used in the making of plaster of Paris for construction (as water will return powdered burnt gypsum to its original rock form), as a retarder in Portland cement, as a flux for pottery and as a fertilizer. It is also used as a filler in crayons, paints, and paper. The low hardness of gypsum precludes its use to any extent except as beads or slabs, though most alabaster is found to be harder. Some varieties of alabaster are used in interior decorating and for sculpture.