Rock Gypsum is a massive, fine-grained variety of the mineral gypsum, widely known as alabaster when translucent. It forms in extensive sedimentary evaporite beds and is easily identified by its low hardness, which allows it to be scratched by a fingernail. It is widely collected for its aesthetic massive forms and historical significance in carving.
Is this rock 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 rock gypsum with a known reference. Rock Gypsum sits at Mohs 2 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Rock Gypsum leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Rock Gypsum typically shows a pearly luster.
- 4Match the color rangeCompare against the expected color range: white, gray, colorless, yellow, brown.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: massive.
Often confused with
Rock 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 3.5 vs. 2); luster reads pearly on Rock Gypsum and vitreous on Anhydrite.
How to tell apart: Calcite is the harder of the two (Mohs 3 vs. 2); luster reads pearly on Rock Gypsum and vitreous on Calcite.
How to tell apart: Rock Gypsum is noticeably harder (Mohs 2 vs. 1).
Often found alongside rock gypsum
Minerals reported to co-occur with rock 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
- 2
- Density
- 2.3 g/cm³
- Streak
- White
- Luster
- Pearly
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Massive
- Cleavage
- Perfect
- Rarity
- Common
- Uses
- Industrial, Decorative, Collector
- Host rock
- Sedimentary Evaporite Beds
- Typical price
- $5-30 specimen
Where rockhounds find rock gypsum
Classic worldwide localities
- Mexico
- USA
- Italy
- Spain
- Canada
Field-hunting tip
Look in sedimentary evaporite beds country — that is the host setting where rock gypsum typically forms. If you start seeing halite, anhydrite, sulfur in float, you are in the right ground. Field specimens usually show a massive habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find rock gypsum on the map
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