RocksdayBlueschist vs Slate
How to tell them apart
A rare blue-gray rock formed at high pressure but low temperature in subduction zones.
Photo: Shannon Heinle, CC0
Shale transformed by low-grade metamorphism into a hard rock that splits into smooth, flat sheets.
Photo: Dexter Perkins, CC0
Quick tests
- Hardness: Blueschist is clearly harder: Mohs 5–6 versus 3–4.
Key differences
- Slate is gray-black, not blue.
Side by side
| Property | Blueschist | Slate |
|---|---|---|
| Type | Metamorphic rock | Metamorphic rock |
| Mohs hardness | 5–6 | 3–4 |
| Luster | Silky to dull | Dull to faint sheen |
| Colors | Blue, Gray | Gray, Black, Green, Purple |
| Crystal system / structure | Foliated, glaucophane-rich | Very fine, foliated (slaty cleavage) |
| Specific gravity | 3–3.2 | 2.7–2.8 |
| Found in | Franciscan Complex, California; Syros, Greece | Gwynedd, Wales; Slate Valley, Vermont–New York |
Properties that are the same for both are grayed out.
How to identify blueschist
- Lavender to blue-gray color.
- Foliated, sometimes with red garnets.
- Found in old subduction zones.
Structure: Foliated, glaucophane-rich
Full Blueschist 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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