RocksdayShale vs Siltstone
How to tell them apart
Compacted mud and clay that splits into thin layers. The most common sedimentary rock.
Photo: James St. John, CC BY 2.0
Rock of silt — grains finer than sand but coarser than clay.
Photo: Mike Davis, CC0
Key differences
- Shale feels smooth on the teeth and splits into thin flakes.
Side by side
| Property | Shale | Siltstone |
|---|---|---|
| Type | Sedimentary rock | Sedimentary rock |
| Mohs hardness | 3 | 3–5 |
| Luster | Dull | Dull |
| Colors | Gray, Black, Red, Green | Gray, Brown, Red |
| Crystal system / structure | Clastic, very fine, splits in layers | Clastic, silt-sized grains |
| Specific gravity | 2.4–2.8 | 2.4–2.7 |
| Found in | Burgess Shale, British Columbia; Marcellus, Pennsylvania | Sedimentary basins worldwide; Grand Canyon (Supai Group) |
Properties that are the same for both are grayed out.
How to identify shale
- Splits into thin, flat flakes along its layers.
- Grains too small to see.
- Smells earthy when wet.
- Soft and crumbly at edges.
Structure: Clastic, very fine, splits in layers
Full Shale guide →How to identify siltstone
- Grains too small to see but gritty against your teeth.
- Thin layers, not as flaky as shale.
- Dull and earthy.
Structure: Clastic, silt-sized grains
Full Siltstone guide →More comparisons
Could you tell them apart from a blurry close-up?
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