RocksdayHornfels vs Slate
How to tell them apart
A dense, very hard rock baked by the heat of nearby magma.
Photo: User Fed on ru.wikipedia, Public domain
Shale transformed by low-grade metamorphism into a hard rock that splits into smooth, flat sheets.
Photo: Dexter Perkins, CC0
Quick tests
- Scratch glass: Hornfels scratches glass (Mohs 6–7); Slate can't (Mohs 3–4).
Key differences
- Slate splits into sheets; hornfels doesn't.
Side by side
| Property | Hornfels | Slate |
|---|---|---|
| Type | Metamorphic rock | Metamorphic rock |
| Mohs hardness | 6–7 | 3–4 |
| Luster | Dull | Dull to faint sheen |
| Colors | Black, Gray, Brown | Gray, Black, Green, Purple |
| Crystal system / structure | Fine-grained, non-foliated | Very fine, foliated (slaty cleavage) |
| Specific gravity | 2.7–3 | 2.7–2.8 |
| Found in | Skiddaw, England; Sierra Nevada, California | Gwynedd, Wales; Slate Valley, Vermont–New York |
Properties that are the same for both are grayed out.
How to identify hornfels
- Dark, fine-grained and very hard.
- Breaks with sharp, splintery edges.
- Rings when tapped.
Structure: Fine-grained, non-foliated
Full Hornfels guide →How to identify slate
- Splits into thin, flat, smooth sheets.
- Rings when tapped.
- Harder than shale.
- Grains invisible.
Structure: Very fine, foliated (slaty cleavage)
Full Slate guide →More comparisons
Could you tell them apart from a blurry close-up?
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