Comparison · Sedimentary Rocks
Mudstone vs Siltstone
Mudstone and Siltstone are easy to confuse because both are sedimentary rocks. Siltstone is harder (Mohs 3.5) than Mudstone (Mohs 2–3), so Siltstone will scratch Mudstone 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 Mudstone and Siltstone
- Siltstone is harder (Mohs 3.5) than Mudstone (Mohs 2–3), so Siltstone will scratch Mudstone 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: Mudstone is Opaque, Siltstone is opaque.
- They overlap in gray, brown, red, and green, but Mudstone also comes in black.
Mudstone vs Siltstone properties
| Property | Mudstone | 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, Brown, Red, Black, Green | Gray, Tan, Brown, Red, Green |
| Rarity | common | common |
| Typical specimen value | $1–$75+ | $1–$75+ |
Which is more valuable?
Typical specimens of Mudstone and Siltstone trade in a similar range, around $10.
About Mudstone
Mudstone is a fine-grained sedimentary rock made of a mixture of silt- and clay-sized particles that has hardened into a blocky mass without the thin, parallel splitting behavior of shale. It sits compositionally between claystone, dominated by clay, and siltstone, which contains coarser silt grains, and forms wherever calm water allows the finest sediment to settle out. Mudstone is one of the most abundant sedimentary rock types in the geologic record.
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.
Mudstone vs Siltstone FAQ
Is Mudstone 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 Mudstone (Mohs 2–3), so Siltstone will scratch Mudstone and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not.
How can you tell Mudstone from Siltstone?
Siltstone is harder (Mohs 3.5) than Mudstone (Mohs 2–3), so Siltstone will scratch Mudstone 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: Mudstone is Opaque, Siltstone is opaque. They overlap in gray, brown, red, and green, but Mudstone also comes in black.
Which is harder, Mudstone or Siltstone?
Siltstone is harder: Mohs 3.5 against Mohs 2–3 for Mudstone.
Which is more valuable, Mudstone or Siltstone?
Typical specimens of Mudstone and Siltstone trade in a similar range, around $10.
