RocksdayChalk vs Diatomite
How to tell them apart
A soft, white limestone made of trillions of microscopic plates shed by ocean algae.
Photo: Nessa Eull, CC0
Chalky white rock made of the glassy shells of microscopic algae called diatoms.
Photo: James St. John, CC BY 2.0
Quick tests
- Acid (vinegar or dilute HCl): Chalk fizzes; Diatomite doesn't.
- Heft: Chalk feels noticeably heavier for its size (specific gravity 1.8–2.3 vs 0.4–0.8).
Key differences
- Chalk fizzes in acid and is heavier.
Side by side
| Property | Chalk | Diatomite |
|---|---|---|
| Type | Sedimentary rock | Sedimentary rock |
| Mohs hardness | 2–3 | 1–1.5 |
| Luster | Dull, earthy | Dull, chalky |
| Colors | White | White, Gray |
| Crystal system / structure | Very fine, microfossil-rich | Microfossil silica (diatoms) |
| Specific gravity | 1.8–2.3 | 0.4–0.8 |
| Found in | White Cliffs of Dover, England; Champagne, France | Lompoc, California; Nevada |
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 diatomite
- White, chalky and extremely light.
- Powders on your fingers.
- Doesn't fizz in acid (unlike chalk).
Structure: Microfossil silica (diatoms)
Full Diatomite guide →More comparisons
Could you tell them apart from a blurry close-up?
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