Gypsum is a soft sulfate mineral commonly found in evaporite sedimentary environments. It is easily identified by its extremely low hardness and ability to form long, transparent bladed crystals known as Selenite, or massive, fine-grained deposits known as Alabaster.
Is this gypsum?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch gypsum with a known reference. Gypsum sits at Mohs 1.5-2, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: subvitreous, pearly on {010} cleavages, silky if fibrous.
- 4Match the color rangeCompare against the expected color range: colorless, white, gray, yellow, honey, yellowish white, greenish white, brown.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: crystalline, massive, tabular.
Often confused with
Gypsum vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Anhydrite is the harder of the two (Mohs 3-3.5 vs. 1.5-2); luster reads subvitreous, pearly on {010} cleavages, silky if fibrous on Gypsum and pearly on {010}, vitreous to greasy on {001}; vitreous on {100} on Anhydrite.
How to tell apart: Calcite is the harder of the two (Mohs 3 vs. 1.5-2); luster reads subvitreous, pearly on {010} cleavages, silky if fibrous on Gypsum and vitreous; pearly on cleavages and {0001} on Calcite.
How to tell apart: Luster reads subvitreous, pearly on {010} cleavages, silky if fibrous on Gypsum and pearly, greasy, dull on Talc.
Often found alongside gypsum
Minerals reported to co-occur with gypsum. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CaSO₄·2H₂O
- Mohs hardness
- 1.5-2
- Density
- 2.317 g/cm³
- Streak
- White
- Luster
- Subvitreous, pearly on {010} cleavages, silky if fibrous
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
- Crystal habit
- Crystalline, massive, tabular
- Cleavage
- {010}, perfect; {100}, distinct; {011}
- Fluorescence
- Orange yellow under SW and LW UV
- Rarity
- Common
- Uses
- Collector, Decorative, Industrial
- Host rock
- A common constituent of sedimentary rocks, particularly marine salt deposits, and soils formed directly by evaporation or later by hydration of anhydrite
Where rockhounds find gypsum
57 mapped spotsClassic worldwide localities
- South Wash, in Wayne Co., Utah, USA
- Great Salt Plains, Alfalfa Co., Oklahoma, USA
- Exceptional speleothems in Lechuguilla Cave, Carlsbad Caverns National Park, New Mexico, USA
- Sicily, as at Racalmuto, Girgenti, and Cianciana, Italy
- Bex, Valais, Switzerland
- Aussee, Styria, and Hall, Tirol, Austria
U.S. states with gypsum
Each link opens a state-specific list of mapped rockhounding spots that produce gypsum.
Field-hunting tip
Where it forms: A common constituent of sedimentary rocks, particularly marine salt deposits, and soils formed directly by evaporation or later by hydration of anhydrite. If you start seeing halite, celestine, calcite in float, you are in the right ground. Typical habit: crystalline, massive, tabular. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Utah, Wyoming, Missouri. Start trip planning there.
More gypsum photos
Common questions
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Find gypsum on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
