RocksdayCalcite vs Gypsum
How to tell them apart
One of the most common minerals on Earth and the main ingredient of limestone and marble. It breaks into perfect slanted boxes (rhombs) and fizzes in acid.
Photo: Robert M. Lavinsky, CC BY-SA 3.0
A soft, water-bearing calcium sulfate formed as seawater and lakes evaporate. Clear crystals are called selenite; sand-filled rosettes are 'desert roses'.
Photo: Didier Descouens, CC BY-SA 4.0
Quick tests
- Fingernail: A fingernail scratches Gypsum (Mohs 2) but not Calcite (Mohs 3).
- Acid (vinegar or dilute HCl): Calcite fizzes; Gypsum doesn't.
Key differences
- Calcite is too hard for a fingernail and fizzes in acid.
Side by side
| Property | Calcite | Gypsum |
|---|---|---|
| Type | Mineral | Mineral |
| Mohs hardness | 3 | 2 |
| Luster | Vitreous | Vitreous to pearly |
| Streak | White | White |
| Colors | Clear, White, Orange, Yellow, Blue, Green | Clear, White, Brown |
| Crystal system | Trigonal | Monoclinic |
| Specific gravity | 2.71 | 2.31–2.33 |
| Formula | CaCO₃ | CaSO₄·2H₂O |
| Found in | Iceland; Chihuahua, Mexico | Naica, Mexico; White Sands, New Mexico |
Properties that are the same for both are grayed out.
How to identify calcite
- Fizzes in cold dilute hydrochloric acid (or slowly in vinegar).
- Breaks into rhomb-shaped pieces along three cleavage planes.
- Hardness 3 — a copper coin barely scratches it, a knife easily does.
- Clear pieces show double images of text underneath.
Crystal system: Trigonal
Full Calcite guide →How to identify gypsum
- Scratches with a fingernail (hardness 2).
- Clear blades that bend slightly before snapping.
- Doesn't fizz in acid.
- Feels warm, not cold like glass.
Crystal system: Monoclinic
Full Gypsum guide →More comparisons
Could you tell them apart from a blurry close-up?
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