RocksdayChalk vs Limestone
How to tell them apart
A soft, white limestone made of trillions of microscopic plates shed by ocean algae.
Photo: Nessa Eull, CC0
A rock made mostly of calcite, often from shells and skeletons of sea creatures. It dissolves to create caves.
Photo: Mike Davis, CC0
Key differences
- Limestone is harder and doesn't leave marks.
- Chalk is white, very soft and leaves marks.
Side by side
| Property | Chalk | Limestone |
|---|---|---|
| Type | Sedimentary rock | Sedimentary rock |
| Mohs hardness | 2–3 | 3–4 |
| Luster | Dull, earthy | Dull |
| Colors | White | White, Gray, Brown |
| Crystal system / structure | Very fine, microfossil-rich | Varied, often fossiliferous |
| Specific gravity | 1.8–2.3 | 2.5–2.7 |
| Found in | White Cliffs of Dover, England; Champagne, France | Bedford, Indiana; Tura, Egypt |
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 limestone
- Fizzes in dilute acid.
- Gray to cream, often with shell fossils.
- A knife scratches it.
- Weathers into smooth, pitted surfaces.
Structure: Varied, often fossiliferous
Full Limestone guide →More comparisons
Could you tell them apart from a blurry close-up?
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