Rock Gypsum

Rock gypsum is a sedimentary rock composed almost entirely of the mineral gypsum (hydrated calcium sulfate). It forms thick beds in evaporite sequences and is one of the most widely mined industrial minerals on Earth, the raw material behind plaster and drywall.

The rock is famously soft, easily scratched by a fingernail in pure varieties, and feels slightly waxy or earthy. Color ranges from pure white through gray, pink, and amber depending on impurities such as clay, iron oxides, or organic matter.

Massive fine-grained gypsum used for carving is called alabaster, while the clear crystalline form is selenite and the fibrous form is satin spar. All are the same mineral species in different habits.

Rock gypsum is a chemical sedimentary rock of evaporitic origin. It precipitates when restricted bodies of seawater or saline lake water evaporate, concentrating dissolved calcium and sulfate ions until gypsum crystallizes out of solution.

It typically forms in arid coastal lagoons, sabkhas (salt flats), and shallow basins with limited inflow. Within an evaporite sequence, gypsum usually precipitates after carbonates but before more soluble salts like halite. Deep burial and heating can dehydrate gypsum into anhydrite, and later rehydration can convert anhydrite back to gypsum.

Major deposits occur in Spain, the United States (Texas, Oklahoma, New Mexico), and around ancient evaporite basins worldwide.

The defining clue is extreme softness: rock gypsum is Mohs 2 and can be scratched easily with a fingernail, distinguishing it from harder white rocks like limestone (which fizzes in acid) and quartzite.

Look for a white to pale gray, sometimes pink or buff color with a sugary, earthy, or fibrous texture. Luster is dull to pearly or satiny in fibrous bands. Streak is white. Unlike calcite-rich rocks, gypsum does not react with dilute hydrochloric acid.

It is lightweight, has a low specific gravity, and may show cleavage flakes in coarser samples. Salty-tasting beds nearby may indicate associated halite.

Rock gypsum is economically vital. Heated (calcined) gypsum loses water to form plaster of Paris, used in drywall, wall plaster, casts, and molds. It is also a key additive in Portland cement, where it controls setting time, and serves as a soil conditioner in agriculture.

Fine alabaster varieties are carved into sculptures, lamps, and ornamental objects. Industrially, gypsum is used in blackboard chalk substitutes, filler in paint and paper, and even in food and pharmaceuticals as a calcium source.

In metaphysical circles, gypsum and its selenite form are associated with clarity and cleansing, though such claims are not scientifically supported.