RocksdayLimestone vs Phosphorite
How to tell them apart
A rock made mostly of calcite, often from shells and skeletons of sea creatures. It dissolves to create caves.
Photo: Mike Davis, CC0
Sedimentary rock rich in phosphate minerals, often with fossil fragments.
Photo: James St. John, CC BY 2.0
Quick tests
- Acid (vinegar or dilute HCl): Limestone fizzes; Phosphorite doesn't.
Key differences
- Limestone fizzes freely and is lighter.
Side by side
| Property | Limestone | Phosphorite |
|---|---|---|
| Type | Sedimentary rock | Sedimentary rock |
| Mohs hardness | 3–4 | 4–5 |
| Luster | Dull | Dull |
| Colors | White, Gray, Brown | Black, Brown, Gray |
| Crystal system / structure | Varied, often fossiliferous | Pellets and nodules of apatite |
| Specific gravity | 2.5–2.7 | 2.6–3 |
| Found in | Bedford, Indiana; Tura, Egypt | Morocco; Bone Valley, Florida |
Properties that are the same for both are grayed out.
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 →How to identify phosphorite
- Dark pellets, nodules or bone fragments.
- Heavier than limestone.
- Often contains shark teeth and bone.
Structure: Pellets and nodules of apatite
Full Phosphorite guide →More comparisons
Could you tell them apart from a blurry close-up?
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