RocksdayFlint vs Obsidian
The quickest way to tell Flint from Obsidian: Flint is clearly harder: Mohs 6.5–7 versus 5–5.5.
Dark nodules of microcrystalline quartz, especially the ones that grew in chalk. In North America the word is also used for any chert that was knapped into tools.
Photo: Otis Crandell, CC BY-SA 3.0
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: Flint is clearly harder: Mohs 6.5–7 versus 5–5.5.
Key differences
- Obsidian is volcanic glass: softer, and thin edges are translucent brown.
Side by side
| Property | Flint | Obsidian |
|---|---|---|
| Type | Sedimentary rock | Igneous rock |
| Mohs hardness | 6.5–7 | 5–5.5 |
| Luster | Waxy | Glassy |
| Colors | Black, Gray, Brown | Black, Brown, Red |
| Crystal system / structure | Microcrystalline quartz nodules | Glassy |
| Specific gravity | 2.5–2.7 | 2.35–2.6 |
| Found in | Chalk cliffs of southern England; Northern France | Glass Buttes, Oregon; Yellowstone, Wyoming |
Properties that are the same for both are grayed out.
How to identify flint
- Dark gray to black nodules, often with a white chalky rind.
- Breaks in smooth curves with razor edges.
- Waxy on a fresh break, and it scratches glass.
- Sparks against steel.
Structure: Microcrystalline quartz nodules
Full Flint 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
Frequently asked questions
What is the difference between flint and obsidian?
Flint is a sedimentary rock with a Mohs hardness of about 6.5–7 and a waxy luster, usually black, gray or brown. Obsidian is an igneous rock with a Mohs hardness of about 5–5.5 and a glassy luster, usually black, brown or red.
How can you tell flint from obsidian?
Flint is clearly harder: Mohs 6.5–7 versus 5–5.5. Obsidian is volcanic glass: softer, and thin edges are translucent brown.
Could you tell them apart from a blurry close-up?
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