Obsidian vs Onyx: How to Tell Them Apart
The quick answer
Look at how it breaks and what is inside: obsidian is glass and chips into smooth, curved shell-like surfaces, while onyx shows flat, straight parallel bands.
Obsidian vs onyx both come in glossy black, but they are completely different materials. Obsidian is natural volcanic glass; onyx is a banded variety of chalcedony (a form of quartz). The fastest tell is structure. Obsidian breaks with a smooth, curved, shell-like fracture (called conchoidal) and may show tiny gas bubbles or swirls inside. Onyx shows flat, parallel color bands and breaks more grainy.
Hardness backs this up: onyx is harder (about 7) and scratches glass, while obsidian is softer (about 5 to 5.5) and will not scratch a glass plate cleanly. For a quick first read from a photo, the crystal identifier can point you to the likelier material.
| Property | Obsidian | Onyx |
|---|---|---|
| Hardness (Mohs) | 5-5.5 (will not scratch glass) | About 7 (scratches glass) |
| Streak | White (glass; rarely tested) | White |
| Luster | Vitreous; bright glassy shine | Vitreous to waxy when polished |
| Color | Usually jet black; also brown, mahogany, or sheen | Black, or black with white bands; many dyed colors |
| Crystal habit / shape | Amorphous glass; no crystals; curved fracture | Microcrystalline quartz in flat parallel bands |
| Density / heft | About 2.4 g/cm3 (feels light) | About 2.6 g/cm3 |
| Price | Low; inexpensive and common | Low to moderate; common, often dyed |
How to tell them apart
Two quick checks separate them almost every time: look at any broken edge, and try a hardness scratch. Obsidian is glass, so a chipped edge is smooth, curved, and shiny like a broken bottle, and it can be razor-sharp. Onyx breaks more grainy and shows its banding on cut faces.
Hardness is the clean confirmation. Onyx is quartz at about 7 and scratches a glass plate; obsidian at about 5 to 5.5 does not. Be careful, since obsidian edges are genuinely sharp. Note that much commercial "black onyx" is solid black with no visible bands because it is dyed, so absence of banding alone does not prove obsidian. Use the fracture and the scratch test together.
- Fracture test: a smooth, curved, shell-like broken surface means obsidian; a grainy break means onyx.
- Banding: flat, straight parallel bands point to onyx; swirls or tiny round gas bubbles point to obsidian.
- Hardness test: onyx (about 7) scratches glass; obsidian (about 5-5.5) does not.
- Heft: obsidian is a touch lighter for its size; onyx feels slightly denser.
- Edge: a freshly chipped edge that is glassy and very sharp is a strong sign of obsidian.
What each one is
Obsidian is volcanic glass. It forms when lava cools so fast that its atoms never organize into crystals, which makes it a mineraloid rather than a true mineral. Because it has no crystal structure, it fractures conchoidally into smooth curved surfaces and extremely sharp edges, a property ancient toolmakers used for blades.
Onyx is a banded form of chalcedony, which is microcrystalline quartz (SiO2). Its defining feature is flat, parallel layering. Pure natural onyx alternates black and white bands, though much of the solid-black onyx sold today is chalcedony or agate that has been dyed. As quartz, it sits at about 7 on the Mohs scale.
Which is more valuable
Both are inexpensive, and neither is a high-value stone. They are common, widely available, and priced as affordable decorative and jewelry materials rather than fine gems. Value comes mostly from carving quality, polish, and size rather than the material itself.
Between the two, onyx is sometimes priced a little higher because it is harder, takes a durable polish, and is popular for cabochons and inlay. Obsidian's appeal is its glassy black depth and varieties like rainbow or snowflake obsidian. If you want to know which one a polished black piece actually is, the fracture and scratch tests above are the reliable way to tell.
Frequently asked questions
How do I tell obsidian from onyx?
Check a broken edge and test hardness. Obsidian is volcanic glass and chips into smooth, curved, shell-like surfaces; onyx breaks grainy and shows flat parallel bands. Onyx is harder at about 7 and scratches glass, while obsidian at about 5 to 5.5 does not.
Is black onyx the same as obsidian?
No. Black onyx is a banded variety of chalcedony (quartz), while obsidian is natural volcanic glass. They can look alike when polished, but onyx is harder and crystalline, and obsidian is amorphous glass with a curved fracture. Much solid-black onyx is also dyed chalcedony.
Does onyx scratch glass and obsidian does not?
Yes. Onyx is quartz at about 7 on the Mohs scale, so it scratches a glass plate. Obsidian is softer at about 5 to 5.5 and will not scratch glass cleanly. It is a quick, reliable way to separate them, though obsidian edges are sharp, so handle them with care.
Why is my black stone perfectly solid with no bands?
It could be either. Obsidian is naturally solid black with no bands, but much commercial "black onyx" is dyed to a uniform black too. So a lack of banding does not prove obsidian. Use the fracture test and a hardness scratch to confirm which material it really is.
Is obsidian sharper than onyx?
Far sharper. Because obsidian is glass with no crystal structure, a fresh fracture produces edges finer than a steel scalpel, which is why it was used for ancient blades. Onyx is crystalline quartz and breaks into grainy, blunter edges. A glassy, razor-sharp chip points to obsidian.
In the field guide
Last updated 2026-06-26. Educational comparison — confirm an identification with the tests described or a qualified expert before relying on it.