RocksdayCoal vs Obsidian
How to tell them apart
A rock formed from buried, compressed plant material — ancient swamp forests turned to stone.
Photo: Amcyrus2012, CC BY 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 2–3.
- Heft: Obsidian feels noticeably heavier for its size (specific gravity 2.35–2.6 vs 1.1–1.8).
Key differences
- Obsidian is glass: harder, heavier and translucent at the edges.
- Coal is far lighter, softer and leaves black marks on your fingers.
Side by side
| Property | Coal | Obsidian |
|---|---|---|
| Type | Sedimentary rock | Igneous rock |
| Mohs hardness | 2–3 | 5–5.5 |
| Luster | Dull to submetallic | Glassy |
| Colors | Black, Brown | Black, Brown, Red |
| Crystal system / structure | Organic, layered | Glassy |
| Specific gravity | 1.1–1.8 | 2.35–2.6 |
| Found in | Powder River Basin, Wyoming; Appalachia | Glass Buttes, Oregon; Yellowstone, Wyoming |
Properties that are the same for both are grayed out.
How to identify coal
- Black and very light for a rock.
- Leaves black dust on hands.
- Layered, with blocky or curved breaks.
- Burns.
Structure: Organic, layered
Full Coal 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?
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