Real black obsidian is volcanic glass, and it behaves like glass in ways that are easy to spot once you know what to look for. It has a glassy shine, feels cool to the touch, breaks with smooth curved edges, and contains tiny natural imperfections that man-made glass and dyed stones simply don’t have. With a few simple tests, you can separate genuine obsidian from the most common fakes.
What Real Obsidian Actually Is
Obsidian forms when silica-rich lava cools so quickly that mineral crystals don’t have time to develop. It contains about 70 percent or more silicon dioxide, the same compound found in granite, but because the magma chills rapidly at or near the Earth’s surface, the result is a natural glass rather than a crystalline rock. This is the key to understanding every test below: real obsidian is glass, but it’s glass with a volcanic history baked into its structure.
Check the Surface and Luster
Genuine black obsidian has what geologists call a vitreous luster, meaning it looks like glass. When polished, the surface is highly reflective, almost mirror-like. When raw or freshly broken, it still has a sharp, glassy sheen rather than a dull, waxy, or chalky appearance.
If the stone in front of you looks flat and matte, it’s more likely black onyx, dyed agate, or another mineral entirely. Onyx is a banded chalcedony that rates 6.5 to 7 on the Mohs hardness scale and typically has a softer, more muted finish compared to obsidian’s bright glassiness. Real obsidian at 5 to 5.5 on the Mohs scale is actually softer than onyx, but it looks shinier because of its glassy structure.
Look for Natural Imperfections
This is the single most useful test you can do at home. Hold the stone up to a strong light source or use a basic magnifying glass (10x is enough). You’re looking for two things: what’s there and what shouldn’t be.
Natural obsidian often contains tiny flow lines, thin streaks or bands created as the viscous lava moved before it solidified. You may also see microscopic mineral inclusions, tiny needle-like crystals called microlites that formed as the glass began to cool. In some varieties, trapped gas and ash created tiny elongated channels during rapid cooling. When these channels are the right size and orientation, they reflect light as a golden or silvery sheen (this is what gives “sheen obsidian” its signature glow).
Man-made glass, by contrast, tends to contain perfectly round gas bubbles. If you see numerous small, spherical bubbles evenly distributed throughout the stone, that’s a strong sign you’re looking at manufactured glass rather than natural obsidian. Natural obsidian can contain some gas inclusions, but they’re typically irregular and elongated rather than perfectly round.
The Temperature Test
Pick up the stone and hold it against your cheek or the inside of your wrist. Real obsidian, like all natural stone and glass, feels noticeably cool to the touch and takes a moment to warm up to your body temperature. Plastic imitations warm almost instantly because plastic doesn’t conduct heat the same way. This test won’t distinguish obsidian from other stones or from man-made glass, but it quickly eliminates plastic fakes, which are more common than you might expect in cheap jewelry and crystal shops.
The Scratch Test
Obsidian sits at 5 to 5.5 on the Mohs hardness scale, according to the International Gemological Institute. That means a steel knife blade (about 5.5) can scratch it with some effort, and obsidian will easily scratch a copper coin (about 3.5). If the stone is too hard to scratch with a knife, it’s likely something else, possibly black onyx, black tourmaline, or dyed glass that’s been tempered.
If you’re willing to test an inconspicuous spot, drag the stone across a piece of unglazed porcelain tile (a streak plate). Obsidian typically leaves no colored streak or a very faint white one. A black streak would suggest a different mineral entirely.
Check the Edges and Fracture Pattern
When obsidian breaks, it fractures in smooth, curved, shell-like patterns called conchoidal fractures. This is the same way window glass breaks, producing sharp edges with a characteristic swooping curve. Ancient peoples used this property to make cutting tools, and it’s one of the reasons obsidian edges can be sharper than surgical steel.
If your piece has any chipped areas or broken edges, examine them closely. You should see those smooth, curved fracture surfaces. A stone that breaks with flat, angular faces is crystalline and not obsidian. A stone that crumbles or breaks with rough, granular surfaces is also something else.
The Weight Test
Obsidian has a specific gravity of 2.35 to 2.60, according to the International Gem Society. In practical terms, it feels about the same weight as a piece of regular glass of the same size, and noticeably heavier than plastic. If you have a kitchen scale and a cup of water, you can do a simple displacement test: weigh the stone dry, then weigh it suspended in water. Divide the dry weight by the difference between the two weights. A result between 2.35 and 2.60 is consistent with obsidian. This test is most useful when you’re trying to distinguish obsidian from plastic or resin fakes, which are significantly lighter.
Common Materials Sold as Black Obsidian
Knowing what fakes look like helps as much as knowing what real obsidian looks like. Here are the most frequent impostors:
- Man-made glass: The most common fake. It looks very similar to obsidian but contains round gas bubbles visible under magnification and lacks the natural flow lines and mineral inclusions of volcanic glass. It’s often perfectly uniform in color with no variation whatsoever.
- Black onyx: A genuine stone, but not obsidian. Onyx has visible banded layers of black and white or brown running through it, a more matte finish, and is harder than obsidian (6.5 to 7 on the Mohs scale). If you can see banding, it’s onyx.
- Dyed glass or resin: Sometimes sold in the form of spheres, skulls, or other carved shapes. Resin is warm to the touch and much lighter than real obsidian. Dyed glass may look right at first glance but often has an overly uniform, “too perfect” appearance.
- Black tourmaline: A crystalline mineral that’s harder than obsidian and has a different internal structure. Under magnification, tourmaline shows long, parallel striations along its length rather than the smooth, glassy interior of obsidian.
Red Flags When Buying
Price is a practical signal. Genuine black obsidian is relatively abundant and affordable. A small tumbled stone should cost a few dollars at most. If you’re seeing prices that seem suspiciously low even for obsidian, the material may be manufactured glass or resin. On the other hand, if a seller charges a premium and claims the stone has rare properties, be skeptical without some verifiable detail about where it was sourced.
Be especially cautious with brightly colored “obsidian” marketed as blue, green, or red obsidian. While natural obsidian does come in varieties with sheens and color plays caused by aligned mineral inclusions that refract light, vivid, uniform colors throughout the entire stone are a hallmark of dyed or manufactured glass. The EPI gemological institute has documented cases of artificial glass marketed as “red obsidian” that was identifiable only by the perfectly round gas bubbles visible under magnification.
If you’re buying online, ask the seller for close-up photos under strong light. Look for the flow lines, slight color variations, or natural inclusions that confirm a volcanic origin. A completely flawless, perfectly uniform black surface with no internal features at all is worth questioning.