Gypsum
Gypsum is a soft, common evaporite mineral, calcium sulfate dihydrate with the formula CaSO4·2H2O, that defines hardness 2 on the Mohs scale and can be scratched with a fingernail. It precipitates from evaporating saline water and occurs in forms ranging from transparent selenite crystals to fibrous satin spar and massive alabaster. Gypsum is also the principal raw material used to manufacture plaster of Paris and drywall.
Gypsum properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | CaSO4·2H2O |
| Mohs hardness | 2 |
| Crystal system | Monoclinic |
| Luster | Vitreous, pearly, or silky (fibrous), earthy when massive |
| Streak | White |
| Cleavage | Perfect, one direction; distinct in two others |
| Fracture | Conchoidal to splintery/fibrous |
| Specific gravity | 2.3–2.35 |
| Transparency | Transparent to translucent |
| Common colors | Colorless, White, Gray, Yellow, Brown, Pink |
| Rarity | common |
Geology & background
Gypsum precipitates directly from evaporating seawater or saline lake water in arid basins, forming thick sedimentary beds interlayered with limestone, shale, and rock salt in classic evaporite sequences. Spectacular examples of its crystal habits include the massive, room-sized selenite crystals of Mexico's Naica Cave, formed by slow mineral-rich groundwater over hundreds of thousands of years, and the vast gypsum sand dunes of White Sands National Park in New Mexico, the largest gypsum dune field on Earth.
Rockhounds should handle gypsum specimens carefully because its softness and perfect cleavage make it prone to scratching and chipping, and selenite's thin, flexible cleavage sheets can peel apart easily. The fibrous satin spar variety shows a silky chatoyant sheen when properly oriented, while alabaster's fine-grained massive form has been carved for sculpture since antiquity. Because gypsum is water-soluble over geologic time, it readily dissolves to form caves and sinkholes in regions with gypsum bedrock.
How to identify Gypsum
Gypsum is soft enough to scratch with a fingernail, a defining hardness-2 property that immediately separates it from most other minerals, and shows a white streak and vitreous to pearly or silky luster depending on the variety. It has perfect cleavage in one direction, producing thin flexible sheets in the selenite variety, and colors range from colorless and white to gray, yellow, or brown. Habits include tabular crystals, fibrous satin spar, and massive alabaster. Gypsum is sometimes confused with calcite, but calcite is harder and reacts with acid, while gypsum does not.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Gypsum is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Gypsum worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Giant selenite crystals in Mexico's Naica Cave, some over ten meters long, are the largest natural crystals ever documented.
- White Sands National Park in New Mexico contains the world's largest field of gypsum sand dunes.
- Gypsum is soft enough to be scratched by a fingernail, making it the reference mineral for hardness 2 on the Mohs scale.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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