Identification guide · Igneous Rocks
How to Identify Obsidian
Obsidian is usually black, brown, and green with a vitreous luster and is translucent. It has a Mohs hardness of 5.5. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. Its streak is white. It is most often confused with Rhyolite, Rainbow Obsidian, and Andesite; the tests below separate them.
Obsidian identification checklist
| Color | Black, Brown, Green, Rainbow | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Vitreous | Fresh surfaces shine like glass; a dull surface usually means weathering, so chip or wet a corner. |
| Transparency | Translucent | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 5.5 | It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. |
| Streak | White | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | None | |
| Fracture | Conchoidal | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 2.4 | Ordinary rock-like heft (specific gravity about 2.4), similar to quartz. |
| Crystal system | Amorphous |
Extra tests for Obsidian
Glass check. Obsidian shares a glassy luster and conchoidal fracture with man-made glass. Glass sits near Mohs 5.5 and often contains round gas bubbles, so a hardness test and a hand lens separate them.
Obsidian look-alikes and how to tell them apart
Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Obsidian vs Rhyolite →
Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Obsidian vs Rainbow Obsidian →
Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Obsidian vs Andesite →
Both sit near Mohs 5.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
Field tips
- Check whether the specimen's color and texture match typical Obsidian examples.
- Use hardness, streak, luster, grain size, and locality together; no single trait is enough for every specimen.
Obsidian identification FAQ
How can you tell if a rock is Obsidian?
Check the color (black, brown, green, and rainbow), the vitreous luster, a Mohs hardness of 5.5 and a white streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Obsidian?
Obsidian has a Mohs hardness of 5.5. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will.
What is the difference between Obsidian and Rhyolite?
Obsidian (Mohs 5.5, vitreous luster) and Rhyolite (Mohs 5.5–7, dull to vitreous (glassy groundmass) luster) look alike, but hardness, streak and heft usually separate them. See the full Obsidian vs Rhyolite comparison for a side-by-side table.
Is Obsidian worth anything?
Entry-level Obsidian specimens sell for around $5, typical collector pieces about $20, and premium examples $100 or more depending on size, quality and locality.
What color is Obsidian?
Obsidian is most often black, brown, green, and rainbow. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
