Comparison · Sedimentary Rocks
Claystone vs Mudstone
Claystone and Mudstone are easy to confuse because both are sedimentary rocks. Both sit near Mohs 2–3, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Below is a side-by-side comparison of their properties and the quick tests that separate them.
Key differences between Claystone and Mudstone
- Both sit near Mohs 2–3, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
- They overlap in gray, red, brown, and green, but Mudstone also comes in black.
Claystone vs Mudstone properties
| Property | Claystone | Mudstone |
|---|---|---|
| Type | Sedimentary Rocks | Sedimentary Rocks |
| Mohs hardness | 2–3 | 2–3 |
| Luster | Dull, earthy | Dull, earthy |
| Transparency | Opaque | Opaque |
| Specific gravity | 2.0–2.6 | 2.0–2.6 |
| Common colors | Gray, Red, Brown, Tan, Green | Gray, Brown, Red, Black, Green |
| Rarity | common | common |
| Typical specimen value | $1–$75+ | $1–$75+ |
Which is more valuable?
Typical specimens of Claystone and Mudstone 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 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.
Claystone vs Mudstone FAQ
Is Claystone the same as Mudstone?
No. Both are sedimentary rocks, but they are distinct rock types with different hardness, streak or luster. Both sit near Mohs 2–3, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
How can you tell Claystone from Mudstone?
Both sit near Mohs 2–3, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. They overlap in gray, red, brown, and green, but Mudstone also comes in black.
Which is harder, Claystone or Mudstone?
They are about equally hard, both near Mohs 2–3.
Which is more valuable, Claystone or Mudstone?
Typical specimens of Claystone and Mudstone trade in a similar range, around $10.
