Short answer: black obsidian is volcanic glass: rhyolitic lava that cooled too fast to crystallize, Mohs about 5 to 5.5, mirror-bright, with a shell-shaped break and often sheen or bubbles inside. Black onyx is chalcedony, microcrystalline quartz, Mohs 7, with a waxy-to-glassy polish and no sheen, and solid black onyx is almost always dyed. Onyx scratches glass; obsidian does not. Obsidian is also lighter for its size.
What is each one actually made of?
The names suggest two black stones. Mineralogically they are a glass and a quartz.
Obsidian is a rock, not a mineral. Geology.com defines it as an igneous rock that forms when molten rock cools so rapidly that atoms are unable to arrange themselves into crystals (Geology.com). The International Gem Society gives its composition as roughly 66 to 72 percent silica plus oxides of calcium, sodium, potassium and other elements, with a basaltic version near 50 percent (IGS obsidian). Because it is a glass, it is unstable over geologic time: Geology.com notes that some obsidian slowly begins to crystallize, which is how the white radial clusters in snowflake obsidian form.
Onyx, in the gemological sense, is chalcedony, the microcrystalline form of quartz. Minerals.net gives it the formula SiO2, hardness 7 and specific gravity 2.63 to 2.65, and defines it as solid black chalcedony or black and white chalcedony with roughly parallel bands (Minerals.net onyx). Chalcedony never forms visible crystals; it grows as nodules, veins and crusts, layer by layer (Minerals.net chalcedony). Banded calcite sold as "Mexican onyx" is a third material entirely, covered below and in our onyx guide.
What are the quickest checks at the counter?
Look for a sheen or a bubble
Tilt the stone under one bright light. A golden, silver or rainbow sheen means obsidian; onyx never shows one. Under a loupe, the IGS lists elongated, torpedo-shaped, round and teardrop-shaped bubbles as typical obsidian inclusions, along with crystallites and needles. Chalcedony has no gas bubbles.
Examine a chip
Obsidian has the best example of conchoidal fracture, as the IGS puts it: chips are smooth, shell-shaped scoops with ripple marks and razor edges, and the broken surface is as glassy as the polished one. Onyx also fractures conchoidally, but its break looks duller, slightly grainy and waxy, because chalcedony is made of interlocking microscopic quartz fibers.
Compare the luster
Polished obsidian looks like a black mirror, with a wet, liquid reflection. The IGS describes chalcedony's polish luster as greasy to vitreous, so good black onyx looks glossy but slightly softer and satin-like next to obsidian.
Feel the weight
The IGS gives typical obsidian a specific gravity of 2.33 to 2.42; Minerals.net gives onyx 2.63 to 2.65. That 10 percent difference is noticeable with two beads or spheres of the same size, one in each palm. It is also a quick check against black glass imitations, which vary widely.
Try the scratch, carefully
Onyx at 7 scratches ordinary window glass. Obsidian, at 5 to 5.5 according to Geology.com, usually will not, or only barely. Test only on an unseen edge, or skip it on finished jewelry. The reverse test also works: a hard steel file catches on obsidian but skates over onyx.
Look for banding
Natural black onyx often carries straight white bands, and Minerals.net notes that the black-and-white banded form is usually natural and not dyed. Obsidian can be streaky, like reddish brown mahogany obsidian, but its streaks are flow lines that swirl and fold, not flat parallel layers.
How do the numbers line up?
| Property | Black obsidian | Black onyx (chalcedony) |
|---|---|---|
| Material | Natural volcanic glass, amorphous | Microcrystalline quartz |
| Composition | About 66 to 72% SiO2 plus oxides | SiO2 |
| Mohs hardness | About 5 to 5.5 | 7 |
| Specific gravity | 2.33 to 2.42 typical | 2.63 to 2.65 |
| Refractive index | 1.48 to 1.51, usually 1.49 | About 1.54 to 1.55 |
| Optics | Isotropic | Microcrystalline, slightly birefringent |
| Fracture | Conchoidal, razor sharp | Conchoidal to uneven, duller |
| Phenomena | Gold, silver and rainbow sheen possible | None |
| Color source | Natural (magnetite and other particles) | Usually dye in solid black stones |
| Typical sources | Volcanic regions: western US, Mexico, Armenia | India, Brazil, Uruguay |
The IGS notes that the specific gravity ranges of chalcedony and obsidian can overlap at the edges, but a refractive index test separates them cleanly. Any gem lab, or a jeweler with a refractometer, can make the call in a minute.
Is black onyx natural or dyed?
Mostly dyed. The IGS describes black onyx as occurring in nature only in thin bands and states that what is found in stores is almost always dyed. Chalcedony is porous enough to take dye, the IGS explains, and the treatment is stable and centuries old. Minerals.net agrees that black onyx is often dyed. That is not a reason to avoid it: dyed black onyx is durable and attractive. It is a reason to be skeptical of a "natural untreated black onyx" premium.
Obsidian is the opposite case. Natural obsidian's color is its own. The fakes are on the other side: the IGS warns that manufactured blue, green or red glass is sold as obsidian, and plain black glass can stand in for black obsidian too. Bright colors with no natural explanation are glass; our obsidian guide covers the varieties that are real.
Where does each come from?
Obsidian forms wherever silica-rich lava erupted, so it is found in present and former volcanic areas. Yellowstone has the most famous American source. The National Park Service describes Obsidian Cliff, a National Historic Landmark since 1996, as exposing about 98 feet (30 m) of obsidian, quarried for toolmaking for more than 11,000 years (NPS). Its obsidian traveled along trade routes from western Canada to the Great Lakes and Ohio, and it makes up about 90% of the obsidian in Hopewell mortuary sites in the Ohio River Valley.
Chalcedony forms at low temperature from silica-rich water filling cavities and veins. Minerals.net lists India, Brazil and Uruguay as onyx sources, the same countries that supply much of the world's agate. The planned black stone identification guide sets both against other black candidates such as schorl and hematite.
Which one wears better?
Onyx, clearly. Geology.com warns that obsidian's hardness of about 5.5 makes it easy to scratch and that it lacks toughness, chipping on impact, so it suits pendants and earrings better than rings and bracelets. The IGS adds that household dust, at hardness 7, exceeds obsidian, so it should be wiped with a soft lint-free cloth rather than scrubbed. Onyx at 7 survives daily wear in rings and beads. For carvings and display pieces, the softer obsidian is actually easier to work, which is why obsidian figurines are inexpensive.
How do prices compare?
Both are inexpensive materials. Obsidian is abundant, so price follows size, polish quality and sheen: plain black tumbles and small carvings cost little, while strong gold or rainbow sheen pieces cost more. Black onyx beads and cabochons are cheap because the dyed material is plentiful; natural banded onyx and cameo-grade layered stone cost more. The trap is the label. "Onyx" carvings from Mexico, as Minerals.net notes, are often banded travertine or calcite, a much softer stone that is not chalcedony at all.
In crystal traditions both are protective stones, obsidian more associated with grounding and onyx with self-control; those are beliefs, not tests.
Which should you buy?
- Choose obsidian for carvings, spheres and display pieces, for sheen varieties (snowflake, rainbow, gold), and for a natural, undyed black.
- Choose black onyx for rings, bracelets and beads that will be worn hard, and accept that solid black stones are dyed.
- Choose banded onyx or sardonyx if you want natural chalcedony layers, the traditional cameo material.
Shopping obsidian and onyx we stock
Check that the listing names the material type. "Onyx" from Mexico is almost always banded calcite, not chalcedony, which matters for care and wear. For obsidian, close photos should show either a clean mirror polish or the sheen the listing claims.
- Black Obsidian Raven / Crow Figurine: a small carved obsidian bird, a good example of how cleanly the glass carves and polishes.
- Black Obsidian Crystal Sphere Polished Volcanic Glass: a polished sphere for judging the mirror luster and checking for sheen under a single light.
- Snowflake Obsidian: obsidian with white crystallized clusters, which no onyx can show.
- Sardonyx palm stone: banded chalcedony onyx in red-brown and white, the natural layered form of the material.
- Carved Onyx Aztec Mayan Tiki Sculpture: about 7 inches and 4 lb of Mexican onyx, the banded calcite type, useful for seeing how differently it carves and feels.
Digital Towns Market
Obsidian and onyx in stock
Frequently asked questions
Is black onyx the same as black obsidian?
No. Black obsidian is volcanic glass formed when lava cools too fast to crystallize. Black onyx is chalcedony, a microcrystalline quartz, usually dyed black. Onyx is harder (7 against about 5.5), heavier for its size and never shows sheen or gas bubbles.
Which is better for a ring, obsidian or onyx?
Onyx. At Mohs 7 it resists scratches from everyday dust and wear, while obsidian at about 5.5 scratches and chips easily. Obsidian works well in pendants and earrings, which take fewer knocks, and in carvings kept on a shelf.
Is black onyx always dyed?
Not always, but solid black onyx usually is. Natural black chalcedony occurs only in thin bands, so most uniform black onyx in shops has been dyed, a stable, long-established treatment. Black and white banded onyx is more often natural.
Can obsidian be fake?
Yes. Manufactured glass is sold as obsidian, especially in bright blue, green or red colors that natural obsidian does not show. Natural obsidian may contain crystallites, needles, sheen or snowflake clusters; perfectly uniform, bubble-free colored "obsidian" is usually factory glass.
Why is Mexican onyx so soft?
Because it is not chalcedony. "Mexican onyx" and "onyx marble" are trade names for banded calcite or travertine, around Mohs 3, which a knife scratches easily. True onyx is chalcedony at Mohs 7. The banding looks similar, but the stones need different care.
Sources
- Geology.com, Obsidian: https://geology.com/rocks/obsidian.shtml
- International Gem Society, Obsidian Value, Price, and Jewelry Information: https://www.gemsociety.org/article/obsidian-jewelry-and-gemstone-information/
- International Gem Society, Chalcedony Value, Price, and Jewelry Information: https://www.gemsociety.org/article/chalcedony-jewelry-and-gemstone-information/
- Minerals.net, The gemstone onyx: https://www.minerals.net/gemstone/onyx_gemstone.aspx
- Minerals.net, The mineral chalcedony: https://www.minerals.net/mineral/chalcedony.aspx
- National Park Service, Obsidian Cliff, Yellowstone National Park: https://www.nps.gov/yell/learn/historyculture/obsidiancliff.htm