Comparison · Metamorphic Rocks
Hornfels vs Slate
Hornfels and Slate are easy to confuse because both are metamorphic rocks. Hornfels is harder (Mohs 6) than Slate (Mohs 3.5), so Hornfels will scratch Slate and not the other way round. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not. Below is a side-by-side comparison of their properties and the quick tests that separate them.
Key differences between Hornfels and Slate
- Hornfels is harder (Mohs 6) than Slate (Mohs 3.5), so Hornfels will scratch Slate and not the other way round. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not.
- Streak color differs: Hornfels leaves a variable; usually pale gray streak on unglazed porcelain, Slate leaves variable; test individual minerals.
- They overlap in gray, black, green, and red, but Hornfels also comes in brown and white and Slate also comes in purple.
Hornfels vs Slate properties
| Property | Hornfels | Slate |
|---|---|---|
| Type | Metamorphic Rocks | Metamorphic Rocks |
| Mohs hardness | 6 | 3.5 |
| Luster | dull | dull |
| Streak | Variable; usually pale gray | Variable; test individual minerals |
| Transparency | opaque | opaque |
| Specific gravity | 2.7 | 2.8 |
| Crystal system | varies | varies |
| Common colors | Gray, Black, Brown, Green, Red, White | Gray, Black, Green, Purple, Red |
| Rarity | common | common |
| Typical specimen value | $1–$75+ | $1–$100+ |
Which is more valuable?
Typical specimens of Hornfels and Slate trade in a similar range, around $10.
About Hornfels
Hornfels is a hard, dense, fine-grained contact metamorphic rock. It often looks dark, tough, and baked, with little visible layering.
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.
Hornfels vs Slate FAQ
Is Hornfels the same as Slate?
No. Both are metamorphic rocks, but they are distinct rock types with different hardness, streak or luster. Hornfels is harder (Mohs 6) than Slate (Mohs 3.5), so Hornfels will scratch Slate and not the other way round. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not.
How can you tell Hornfels from Slate?
Hornfels is harder (Mohs 6) than Slate (Mohs 3.5), so Hornfels will scratch Slate and not the other way round. A steel knife blade (Mohs 5.5) scratches it; a copper coin does not. Streak color differs: Hornfels leaves a variable; usually pale gray streak on unglazed porcelain, Slate leaves variable; test individual minerals. They overlap in gray, black, green, and red, but Hornfels also comes in brown and white and Slate also comes in purple.
Which is harder, Hornfels or Slate?
Hornfels is harder: Mohs 6 against Mohs 3.5 for Slate.
Which is more valuable, Hornfels or Slate?
Typical specimens of Hornfels and Slate trade in a similar range, around $10.
