Gypsum

Gypsum is a soft, common evaporite mineral made of hydrated calcium sulfate. It is the index mineral for hardness 2 on the Mohs scale — soft enough to scratch with a fingernail.

Gypsum appears in several distinct forms: transparent, bladed crystals called selenite; the silky, fibrous satin spar; fine-grained massive alabaster used for carving; and the bladed rosettes known as desert rose.

It is one of the most widely used industrial minerals on Earth, the basis of plaster of Paris and drywall (gypsum board), and a key ingredient in cement.

Gypsum forms primarily as an evaporite — it precipitates when seawater or saline lake water evaporates in restricted basins, concentrating dissolved calcium and sulfate. Thick gypsum beds mark ancient evaporating seas.

It also forms by the hydration of anhydrite (anhydrous calcium sulfate), in volcanic fumaroles, and from the oxidation of sulfide minerals reacting with limestone. Desert roses crystallize in sandy soils where mineral-rich water evaporates around sand grains.

Major deposits occur worldwide, including the spectacular giant selenite crystals of the Cave of Crystals (Naica) in Mexico and the white gypsum dunes of White Sands, New Mexico.

The single best test is hardness: gypsum is so soft (Mohs 2) you can scratch it with a fingernail, which immediately separates it from calcite (3) and quartz (7).

It is typically colorless to white with a vitreous, pearly, or silky luster, and selenite shows excellent cleavage into thin flexible flakes. It does not fizz in acid, unlike calcite.

Look-alikes: Calcite (harder, fizzes in acid), halite (tastes salty, cubic cleavage), and quartz/selenite confusion (quartz is far harder). Satin spar's silky chatoyancy can mimic some fibrous minerals but its softness gives it away.

Gypsum is enormously important industrially. Heated and rehydrated, it becomes plaster of Paris and the gypsum board (drywall) used in nearly all modern construction. It is also added to Portland cement to control setting time and applied to soil as a conditioner and source of calcium and sulfur.

Alabaster has been carved into sculptures, vases, and ornaments since antiquity, and selenite and satin spar are popular with collectors and in the crystal trade.

Metaphysically, selenite is associated with clarity and cleansing, though such claims are not scientifically established. Note that selenite dissolves in water and should be kept dry.