<P> 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 . </P> <P> Selenite may also occur in a silky, fibrous form, in which case it is commonly called "satin spar". Finally, it may also be granular or quite compact . In hand - sized samples, it can be anywhere from transparent to opaque . 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 . It also forms some of the largest crystals found in nature, up to 12 m (39 ft) long, in the form of selenite . </P> <P> Gypsum is a common mineral, with thick and extensive evaporite beds in association with sedimentary rocks . Deposits are known to occur in strata from as far back as the Archaean eon . Gypsum is deposited from lake and sea water, as well as in hot springs, from volcanic vapors, and sulfate solutions in veins . Hydrothermal anhydrite in veins is commonly hydrated to gypsum by groundwater in near - surface exposures . It is often associated with the minerals halite and sulfur . Gypsum is the most common sulfate mineral . Pure gypsum is white, but other substances found as impurities may give a wide range of colors to local deposits . </P> <P> Because gypsum dissolves over time in water, gypsum is rarely found in the form of sand . However, the unique conditions of the White Sands National Monument in the US state of New Mexico have created a 710 km (270 sq mi) expanse of white gypsum sand, enough to supply the construction industry with drywall for 1,000 years . Commercial exploitation of the area, strongly opposed by area residents, was permanently prevented in 1933 when president Herbert Hoover declared the gypsum dunes a protected national monument . </P>

Where does gypsum often form (site 3)