RocksdayJet vs Obsidian
How to tell them apart
A hard, compact black lignite — fossil wood — that takes a mirror polish.
Photo: Geni, 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 2.5–4.
- Heft: Obsidian feels noticeably heavier for its size (specific gravity 2.35–2.6 vs 1.3–1.4).
- Texture: Obsidian is a rock — a mix of grains or minerals; Jet is a single mineraloid.
Key differences
- Obsidian is heavier, harder and cold to the touch.
Side by side
| Property | Jet | Obsidian |
|---|---|---|
| Type | Mineraloid | Igneous rock |
| Mohs hardness | 2.5–4 | 5–5.5 |
| Luster | Velvety to vitreous (polished) | Glassy |
| Streak | Brown | — |
| Colors | Black | Black, Brown, Red |
| Crystal system / structure | Amorphous (organic) | Glassy |
| Specific gravity | 1.3–1.4 | 2.35–2.6 |
| Found in | Whitby, England; Asturias, Spain | Glass Buttes, Oregon; Yellowstone, Wyoming |
Properties that are the same for both are grayed out.
How to identify jet
- Jet black, light and warm to the touch.
- Brown streak.
- Smells like coal when a hot needle touches it.
Structure: Amorphous (organic)
Full Jet 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