RocksdayObsidian vs Onyx
How to tell them apart
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
Chalcedony with straight, parallel bands, classically black and white. Most solid black onyx is dyed.
Photo: W.carter, CC BY 4.0
Quick tests
- Hardness: Onyx is clearly harder: Mohs 6.5–7 versus 5–5.5.
- Texture: Obsidian is a rock — a mix of grains or minerals; Onyx is a single mineral.
Key differences
- Obsidian is softer glass and lacks banding.
Side by side
| Property | Obsidian | Onyx |
|---|---|---|
| Type | Igneous rock | Mineral |
| Mohs hardness | 5–5.5 | 6.5–7 |
| Luster | Glassy | Waxy to vitreous |
| Streak | — | White |
| Colors | Black, Brown, Red | Black, White |
| Crystal system / structure | Glassy | Microcrystalline quartz (chalcedony) |
| Specific gravity | 2.35–2.6 | 2.58–2.64 |
| Formula | — | SiO₂ |
| Found in | Glass Buttes, Oregon; Yellowstone, Wyoming | Brazil; India |
Properties that are the same for both are grayed out.
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 →How to identify onyx
- Straight, parallel bands — not curved like most agate.
- Hard (scratches glass), waxy polish.
- Uniform jet-black stones are usually dyed chalcedony.
Crystal system: Microcrystalline quartz (chalcedony)
Full Onyx guide →More comparisons
Could you tell them apart from a blurry close-up?
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