Comparison · Sedimentary Rocks
Claystone vs Siltstone
Claystone and Siltstone are easy to confuse because both are sedimentary rocks. Siltstone is harder (Mohs 3.5) than Claystone (Mohs 2–3), so Siltstone will scratch Claystone and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not. Below is a side-by-side comparison of their properties and the quick tests that separate them.
Key differences between Claystone and Siltstone
- Siltstone is harder (Mohs 3.5) than Claystone (Mohs 2–3), so Siltstone will scratch Claystone and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not.
- Hold a thin edge to the light: Claystone is Opaque, Siltstone is opaque.
Claystone vs Siltstone properties
| Property | Claystone | Siltstone |
|---|---|---|
| Type | Sedimentary Rocks | Sedimentary Rocks |
| Mohs hardness | 2–3 | 3.5 |
| Luster | Dull, earthy | dull |
| Streak | — | Variable; often light gray to brown |
| Transparency | Opaque | opaque |
| Specific gravity | 2.0–2.6 | 2.45 |
| Crystal system | — | varies |
| Common colors | Gray, Red, Brown, Tan, Green | Gray, Tan, Brown, Red, Green |
| Rarity | common | common |
| Typical specimen value | $1–$75+ | $1–$75+ |
Which is more valuable?
Typical specimens of Claystone and Siltstone trade in a similar range, around $10.
About Claystone
Claystone is a fine-grained sedimentary rock composed of more than two-thirds clay-sized particles, less than 0.0039 millimeters across, that has hardened into a blocky, non-fissile mass rather than splitting into thin sheets. It differs from shale mainly by this lack of fissility, breaking instead into irregular blocky or conchoidal fragments. Claystone forms from the compaction and slight cementation of clay deposited in calm, low-energy water.
About Siltstone
Siltstone is a fine-grained sedimentary rock made mostly of silt-sized particles. It feels smoother than sandstone but grittier than claystone or shale.
Claystone vs Siltstone FAQ
Is Claystone the same as Siltstone?
No. Both are sedimentary rocks, but they are distinct rock types with different hardness, streak or luster. Siltstone is harder (Mohs 3.5) than Claystone (Mohs 2–3), so Siltstone will scratch Claystone and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not.
How can you tell Claystone from Siltstone?
Siltstone is harder (Mohs 3.5) than Claystone (Mohs 2–3), so Siltstone will scratch Claystone and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not. Hold a thin edge to the light: Claystone is Opaque, Siltstone is opaque.
Which is harder, Claystone or Siltstone?
Siltstone is harder: Mohs 3.5 against Mohs 2–3 for Claystone.
Which is more valuable, Claystone or Siltstone?
Typical specimens of Claystone and Siltstone trade in a similar range, around $10.
