Comparison · Sedimentary Rocks vs Metamorphic Rocks
Siltstone vs Slate
Siltstone and Slate are easy to confuse. Both sit near Mohs 3.5, 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 Siltstone and Slate
- Both sit near Mohs 3.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
- Streak color differs: Siltstone leaves a variable; often light gray to brown streak on unglazed porcelain, Slate leaves variable; test individual minerals.
- They overlap in gray, red, and green, but Siltstone also comes in brown and Slate also comes in black and purple.
Siltstone vs Slate properties
| Property | Siltstone | Slate |
|---|---|---|
| Type | Sedimentary Rocks | Metamorphic Rocks |
| Mohs hardness | 3.5 | 3.5 |
| Luster | dull | dull |
| Streak | Variable; often light gray to brown | Variable; test individual minerals |
| Transparency | opaque | opaque |
| Specific gravity | 2.45 | 2.8 |
| Crystal system | varies | varies |
| Common colors | Gray, Tan, Brown, Red, Green | Gray, Black, Green, Purple, Red |
| Rarity | common | common |
| Typical specimen value | $1–$75+ | $1–$100+ |
Which is more valuable?
Typical specimens of Siltstone and Slate trade in a similar range, around $10.
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.
About Slate
Slate is a fine-grained metamorphic rock that splits easily into flat sheets. Its durability and water resistance make it ideal for roofing and flooring.
Siltstone vs Slate FAQ
Is Siltstone the same as Slate?
No. Siltstone is a sedimentary rock while Slate is a metamorphic rock. Both sit near Mohs 3.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
How can you tell Siltstone from Slate?
Both sit near Mohs 3.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Streak color differs: Siltstone leaves a variable; often light gray to brown streak on unglazed porcelain, Slate leaves variable; test individual minerals. They overlap in gray, red, and green, but Siltstone also comes in brown and Slate also comes in black and purple.
Which is harder, Siltstone or Slate?
They are about equally hard, both near Mohs 3.5.
Which is more valuable, Siltstone or Slate?
Typical specimens of Siltstone and Slate trade in a similar range, around $10.
