San Diego Natural History Museum--Your Nature ConnectionSDNHM Field Guide
[Gypsum.  Collection of the San Diego Natural History Museum.]

Gypsum

From the Greek, gypsos, meaning "plaster."

Description and Occurrence

Gypsum is an evaporite, which means its crystals form during the evaporation of water. The crystals are shaped like prisms or flat plates, and can grow up to 1 meter. Gypsum can appear as transparent crystals (selenite); fibrous, elongated crystals (satin spar); granular and compact masses (alabaster); and in rosette-shaped aggregates called desert roses.

Most gypsum is used in the building and agricultural industries. As a building material, it's used in plaster, plaster of Paris, wallboard, cement, and ceramic tiles. In agriculture, it's used as an amendment to neutralize alkaline soil. Some gypsum -- dense and fine-grained -- is called alabaster and can be carved.

Common around the world, gypsum is found primarily in sedimentary rock. In North America, crystals can be found in New York, Utah, and Oklahoma. In this region, gypsum is mined and processed in a major production plant, located in the aptly named Plaster City between Ocotillo and El Centro. The whole area is white with dust.

Gypsum clusters, called desert roses, form in the desert from the evaporation of groundwater. Desert roses can be found near Ocotillo, in the Imperial Valley.

During the Museum's exhibit of Shona sculpture, the artist tried carving with alabaster. The piece turned out magnificently; however, the artist did not like the softness of the material.

Field Notes: Gypsum is very soft, and the crystals are usually flat. Desert roses often incorporate grains of sand.


Physical Properties

Color Streak Transparency Luster Hardness Cleavage Fracture Specific gravity Crystal form
colorless, white, gray, yellow, red, brown white transparent to opaque vitreous, pearly on cleavage 1.5 to 2 perfect in one direction conchoidal, splintery 2.3 to 2.4 monoclinic

Photo: Gypsum. Collection of the San Diego Natural History Museum
Photo credit: Linda West

Field Guide: Minerals | Field Guide Feedback Form