RocksdayAnthracite vs Obsidian
How to tell them apart
The highest-grade coal: hard, shiny and almost pure carbon.
Photo: Jakec, CC BY-SA 4.0
Natural volcanic glass, formed when silica-rich lava cools so fast that crystals have no time to grow.
Photo: Ji-Elle It feels nice and warm It feels like a love storm, CC BY-SA 3.0
Quick tests
- Hardness: Obsidian is clearly harder: Mohs 5–5.5 versus 2.75–3.
- Heft: Obsidian feels noticeably heavier for its size (specific gravity 2.35–2.6 vs 1.3–1.4).
Key differences
- Obsidian is heavier, harder and translucent at edges.
Side by side
| Property | Anthracite | Obsidian |
|---|---|---|
| Type | Metamorphic rock | Igneous rock |
| Mohs hardness | 2.75–3 | 5–5.5 |
| Luster | Submetallic | Glassy |
| Colors | Black | Black, Brown, Red |
| Crystal system / structure | Dense carbon, conchoidal fracture | Glassy |
| Specific gravity | 1.3–1.4 | 2.35–2.6 |
| Found in | Northeastern Pennsylvania; South Wales | Glass Buttes, Oregon; Yellowstone, Wyoming |
Properties that are the same for both are grayed out.
How to identify anthracite
- Shiny, nearly metallic black.
- Breaks with curved, glassy fractures.
- Barely dirties your fingers.
Structure: Dense carbon, conchoidal fracture
Full Anthracite guide →How to identify obsidian
- Shiny, glass-like and usually black.
- Curved, shell-shaped (conchoidal) fractures with razor-sharp edges.
- Thin edges are translucent brown or gray.
- May show white 'snowflakes' or a gold/rainbow sheen.
Structure: Glassy
Full Obsidian guide →More comparisons
Could you tell them apart from a blurry close-up?
Play today's Rocksday