RocksdayChalk vs Gypsum
How to tell them apart
A soft, white limestone made of trillions of microscopic plates shed by ocean algae.
Photo: Nessa Eull, CC0
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
- Acid (vinegar or dilute HCl): Chalk fizzes; Gypsum doesn't.
- Texture: Chalk is a rock — a mix of grains or minerals; Gypsum is a single mineral.
Key differences
- Gypsum doesn't fizz.
Side by side
| Property | Chalk | Gypsum |
|---|---|---|
| Type | Sedimentary rock | Mineral |
| Mohs hardness | 2–3 | 2 |
| Luster | Dull, earthy | Vitreous to pearly |
| Streak | — | White |
| Colors | White | Clear, White, Brown |
| Crystal system / structure | Very fine, microfossil-rich | Monoclinic |
| Specific gravity | 1.8–2.3 | 2.31–2.33 |
| Formula | — | CaSO₄·2H₂O |
| Found in | White Cliffs of Dover, England; Champagne, France | Naica, Mexico; White Sands, New Mexico |
Properties that are the same for both are grayed out.
How to identify chalk
- Bright white and powdery.
- Leaves white marks on hands and surfaces.
- Fizzes in acid.
- Very light and porous.
Structure: Very fine, microfossil-rich
Full Chalk 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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