All varieties of gypsum, including selenite and alabaster, are composed of calcium sulfate dihydrate (meaning that it has two molecules of water), with the chemical formula CaSO4·2H2O.

"Selenite" is mostly synonymous with gypsum,[2] but from the 15th century, it has named the transparent variety that occurs in crystals or crystalline masses.

[4][5] Other distinguishing characteristics include its crystal habits, pearly lustre, easy fusibility with loss of water, and solubility in hot dilute hydrochloric acid.

General identifying descriptions of the related crystalline varieties are: Satin spar is sometimes cut into cabochons to best display its chatoyance.

[9] Selenite crystals sometimes form in thin tabular or mica-like sheets and have been used as window panes[17][18] as at Santa Sabina in Rome.

Both selenite crystals and gypsum flowers sometimes form quite densely in acicular mats or nets; and can be quite brittle and fragile.

[21][22] Gypsum crystals are colorless (most often selenite), white (or pearly – most often satin spar), or gray, but may be tinted brown, yellow, red, or blue by the presence of impurities, such as iron oxides or clay minerals.

[citation needed] Selenite crystals that exhibit in either reticular or acicular habits, satin spar, in general (as fibrous crystals are thin and narrow), desert roses that are thinly bladed, and gypsum flowers, particularly acicular gypsum flowers, can be quite brittle and easily broken.

[citation needed] All four crystalline varieties can range in size from minute to giant selenite crystals measuring 11 meters long such as those found in the caves of the Naica Mine of Chihuahua, Mexico.

[26] In dry, desert conditions and arid areas, sand may become trapped both on the inside and the outside of gypsum crystals as they form.

[27] Exterior inclusion (druse) occurs as embedded sand grains on the surface such as, commonly seen in the familiar desert rose.