About
Gypsum has the same chemical composition as the mineral Anhydrite, but contains water in its structure, which Anhydrite lacks. Many Anhydrite specimens absorb water, transforming into the more common Gypsum. Some Gypsum specimens show evidence of this, containing growths of crumpling layers that testify to their expansion from the addition of water.
In a small number of Gypsum specimens, water gets trapped inside a crystal in a hollow channel while the crystal forms. When such a crystal is rotated, a water "bubble" moves around inside it toward the lowest point in the channel. Such specimens are considered a mineralogical oddity, and are very desirable to collectors. These are called "enhydros".
Gypsum sometimes forms in sandy areas, and crystals may trap sand inside when forming, causing a specimen to become brown or gray and opaque. These sand inclusions sometimes form hourglass formations in a crystal. They are also present in the well-known "Desert Rose", which is rosette shaped Gypsum with sand inclusions. (The term "Desert Rose" also applies to rosette shaped Barite with sand inclusions, and the two should not be confused.)
Gypsum specimens should only be cleaned with water. Soaps and detergents should be avoided, as they can enter cracks and crevices of a crystal and ruin its luster.
Crystal Forms & Aggregates
Other ID Marks
Striking Features
Environment
Varieties
✓ common · ✗ uncommon
- Alabaster— Massive, fine grained variety of Gypsum. Occasionally may also refer to a translucent, banded type of Aragonite in the form of Travertine or Tufa.
- Desert Rose— Rosette shaped Barite or rosette shaped Gypsum with sand inclusions.
- Satin Spar— Fibrous variety of Gypsum. May occasionally also describe a fibrous form of Calcite or Aragonite.
- Selenite— Transparent and colorless (or very lightly colored) variety of Gypsum that forms in distinct crystals.
- Gypsum Flower— Rosette shaped Gypsum found in caverns with spreading fibers.
- Gypsum Rock— Rock composed primarily of the mineral Gypsum, but also contain impurities such as Calcite, Anhydrite, Halite, Dolomite, Limonite, and clay.
- Ram's Horn— Curved Gypsum with a shape similar to a ram's horn.
- Sand Gypsum— Gypsum with inclusions of sand, causing the specimen to be brown or grayish in color and opaque.
Uses
Noteworthy Localities
Exceptional, bright orange crystals of Gypsum come from the salt mine of Las Salinas de Paracas, Pisco, Peru. Mexico boasts the most notable and finest localities of this mineral. Besides for the abundance of specimens that have come from Naica, in Chihuahua, several famous caves have been discovered in these mines bearing the worlds largest known crystals. In 2001, a cave called "Cave of the Crystals" was discovered where gigantic, elongated clear crystals were found. The largest was measured to be 37.4 feet long! (Make sure to view the link to National Geographic link in the "Additional Resources" section below.)
Other important Mexican Gypsum localities are Saltillo, in Coahuila, which provides much of the Gypsum Desert Rose available on the mineral market. These specimens can be identified by their whitish edges. Santa Eulalia, in Chihuahua has produced outstanding Selenite. In Canada, excellent yellow crystals come from the Red River Floodway in Winnipeg, Manitoba. A huge industrial deposit mined for sheet rock is in Windsor, Nova Scotia.
In the U.S., Desert Roses are plentiful in the Great Salt Plains, near Jet, Alfalfa Co., Oklahoma. Some excellent hourglass Sand Gypsum has also come from there. Desert Roses are also plentiful in the Mojave Desert in California. Stand-alone, perfect Selenite crystals come from Ellsworth, Mahoning Co., Ohio. Curved Gypsum crystals and Gypsum Flowers exist in the Mammoth Caves of Kentucky, and large crystals of Selenite have come from Hanksville, Wayne Co., Utah. Curved fibrous masses come from Terlingua, Brewster Co., Texas; and nice clear crystals are found in New York at Lockport, Niagara Co.; and at Kerhonkson, Ulster Co.
Common Mineral Associations
Distinguishing Similar Minerals
Calcite - Harder, not flexible and not sectile.
Barite {%Desert Rose} - Distinguished from Gypsum Desert Rose in that it is harder (3 - 3½) and much heavier (4.3 - 4.6).
Photos
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