RocksdayCalcite vs Selenite
The quickest way to tell Calcite from Selenite: A fingernail scratches Selenite (Mohs 2) but not Calcite (Mohs 3).
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
The clear, bladed variety of gypsum. Thin sheets peel off and bend, and a fingernail scratches it.
Photo: Parent Géry, CC BY-SA 3.0
Quick tests
- Fingernail: A fingernail scratches Selenite (Mohs 2) but not Calcite (Mohs 3).
- Acid (vinegar or dilute HCl): Calcite fizzes; Selenite doesn't.
Key differences
- Calcite is harder and fizzes in acid.
Side by side
| Property | Calcite | Selenite |
|---|---|---|
| Type | Mineral | Mineral |
| Mohs hardness | 3 | 2 |
| Luster | Vitreous | Vitreous to pearly |
| Streak | White | White |
| Colors | Clear, White, Orange, Yellow, Blue, Green | Clear, White, Gray, Brown |
| Crystal system | Trigonal | Monoclinic |
| Specific gravity | 2.71 | 2.3 |
| Formula | CaCO₃ | CaSO₄·2H₂O |
| Found in | Iceland; Chihuahua, Mexico | Great Salt Plains, Oklahoma; Naica, 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 selenite
- Colorless blades or windows you can see through.
- A fingernail scratches it (hardness 2).
- Thin sheets bend and stay bent; they do not spring back like mica.
- Does not fizz in acid and does not taste salty.
Crystal system: Monoclinic
Full Selenite guide →More comparisons
Frequently asked questions
What is the difference between calcite and selenite?
Calcite is a mineral (CaCO₃) with a Mohs hardness of 3 and a vitreous luster, usually colorless, white, orange, yellow, blue or green. Selenite is a mineral (CaSO₄·2H₂O) with a Mohs hardness of 2 and a vitreous to pearly luster, usually colorless, white, gray or brown.
How can you tell calcite from selenite?
A fingernail scratches Selenite (Mohs 2) but not Calcite (Mohs 3). Calcite fizzes; Selenite doesn't. Calcite is harder and fizzes in acid.
Could you tell them apart from a blurry close-up?
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