RocksdayBasalt vs Obsidian
How to tell them apart
The most common volcanic rock on Earth. Dark, fine-grained lava that forms the ocean floor and vast lava plains.
Photo: James St. John, CC BY 2.0
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
Key differences
- Basalt is dull and grainy, never glassy.
Side by side
| Property | Basalt | Obsidian |
|---|---|---|
| Type | Igneous rock | Igneous rock |
| Mohs hardness | 6 | 5–5.5 |
| Luster | Dull | Glassy |
| Colors | Black, Gray | Black, Brown, Red |
| Crystal system / structure | Fine-grained (aphanitic) | Glassy |
| Specific gravity | 2.9–3.1 | 2.35–2.6 |
| Found in | Hawaii; Iceland | Glass Buttes, Oregon; Yellowstone, Wyoming |
Properties that are the same for both are grayed out.
How to identify basalt
- Dark gray to black and heavy.
- Grains too small to see.
- Often has gas bubbles (vesicles).
- May show six-sided columns in outcrops.
Structure: Fine-grained (aphanitic)
Full Basalt 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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