Onyx
Black onyx is treated chalcedony, by a Roman process nobody objects to. What onyx actually is, why onyx marble is not it, and what to pay for it.
Onyx is the plain black stone of jewellery: even, matte-free, deeply glossy, and used for signet rings, cameos and mourning pieces for two thousand years. It is inexpensive, hard enough to wear, and the only stone in this series where the main thing to understand is that the name has quietly changed meaning.
Strictly, onyx is a banded chalcedony with straight parallel layers, classically black and white. What the trade sells as black onyx is almost always a solid black stone with no bands at all, produced by treating grey chalcedony. That treatment is ancient, stable, universal and entirely accepted, but a buyer should know it is there.
This page covers what onyx actually is, the treatment and why nobody objects to it, the difference between onyx and the unrelated material sold as onyx marble, what it costs, and where it belongs in a ring.
What it is
Onyx is chalcedony, which is quartz in microscopically fine fibrous form. Because the crystals are too small to see, chalcedony is translucent to opaque rather than transparent, and it takes an exceptionally smooth polish, which is why it has always been the material of choice for carving.
The banded structure that gives onyx its name comes from silica depositing in layers inside a cavity, each layer slightly different in colour or transparency. In true onyx those bands are straight and parallel, which is what makes it suitable for cameo carving: cut down through a white layer into a black one and the design stands out in relief against a dark ground.
Agate is the same material with curved or concentric banding rather than straight, and sardonyx is onyx whose bands run reddish brown and white rather than black and white. All three are chalcedony and the distinctions are about pattern, not composition.
The treatment, and why it is accepted
Solid black chalcedony is rare in nature. The black onyx sold everywhere is grey or pale banded chalcedony soaked in a sugar solution and then heated in sulphuric acid, which carbonises the sugar within the pores and blackens the stone permanently and evenly.
This process is documented in Roman sources and has been in continuous use for around two thousand years. It is stable, it does not fade, it penetrates the whole stone rather than coating it, and it is so completely standard that a dealer offering black onyx is understood to be offering treated material. It is one of the very few treatments the trade regards as simply part of the product.
Two practical consequences. Because the process is universal, treated onyx carries no price penalty and asking whether it has been treated will get you a slightly puzzled yes. And because the colour is carbon in the pore structure rather than a surface layer, a chip or a scratch does not reveal a pale interior.
Onyx marble is a different material
Vases, lamp bases, chess sets and bathroom tiles are frequently sold as onyx and are usually onyx marble, a banded calcite. It is a completely unrelated material: calcite is 3 on the Mohs scale rather than 7, it is soluble in weak acid, and it is quarried in blocks rather than found as nodules.
The two are not interchangeable and the confusion matters if you are buying a carved piece rather than a cut stone. If a large translucent honey-and-cream banded object is described as onyx, it is almost certainly calcite, and it should not be set into a ring under any circumstances.
Judging it
There is not a great deal to judge in a solid black stone, which is part of its appeal, but four things separate a good one from a poor one:
| Factor | What to look for |
|---|---|
| Evenness | A uniform dense black, with no grey patches or visible remnant banding |
| Polish | A deep glassy shine; onyx should look wet, and a dull finish is a cut fault |
| Surface | No pitting or undercut spots, which show where the chalcedony was porous |
| Symmetry | For cabochons and flat tablets, an even dome and a true outline |
Because there is no colour, clarity or brilliance to argue about, workmanship is nearly the whole story. A well-cut, well-polished onyx costs very little more than a badly cut one and looks substantially better, so there is no reason to accept the latter.
Where it comes from
Chalcedony suitable for treatment is common and widely distributed, with Brazil, India, Madagascar and Uruguay all major suppliers, and most of the cutting and treating done in India, Germany and China. The historic German cutting centre at Idar-Oberstein built much of its reputation on agate and onyx work.
Origin is irrelevant to price. This is an abundant material and no locality commands a premium.
Hardness and everyday wear
Onyx is chalcedony, so it is 6.5 to 7 on the Mohs scale with no cleavage and good toughness. That makes it a practical everyday stone, more durable than opal, moonstone, turquoise or tanzanite, and comparable to tiger's eye.
The one thing that shows wear is the polish. A high gloss on a flat black surface makes every fine scratch visible, and a signet or a flat tablet will haze over years in a way a domed cabochon will not. Repolishing is straightforward and cheap. Our guide to settings covers mountings; a bezel or a flush inlay suits it, and it is one of the classic stones for a flat-topped signet.
Avoid ultrasonic cleaning if the stone is a doublet or is set with adhesive, and keep it away from strong acids, which will not touch the quartz but will attack any calcite mistakenly sold as onyx.
What it costs
Very little, and again worth stating plainly:
| Form | Typical price |
|---|---|
| Calibrated cabochons and tablets | Under $1 per carat |
| Larger well-finished pieces | $1 - $3 per carat |
| Fine hand-carved cameos and intaglios | Priced as carving, from tens to thousands |
The material is essentially free and the value is in the work. This is the one stone here where a four figure price can be entirely legitimate and have nothing to do with the stone itself, because you are buying an engraver's labour rather than a mineral.
Meaning and symbolism
Onyx is not a modern birthstone, though it appears in some older lists for July and occasionally for December. Our astrology and gemstones guide covers how those older assignments worked and why they disagree with the calendar system.
Its strongest cultural association in Europe is with mourning. Victorian jet and onyx jewellery was worn during formal periods of grief, and black stones set in gold carry that echo for many people still. Older traditions treat it as a stone of self-control and steadiness, and Roman soldiers carried onyx intaglios of Mars.
None of this makes it a sombre choice today. Black onyx set against yellow gold reads as clean and graphic rather than funereal, and it has been a staple of modern men's jewellery for exactly that reason.
Designing around onyx
- It is the classic signet stone. A flat oval tablet in a heavy bezel is the traditional form and still the best one. Our settings guide covers the options.
- Expect to repolish a flat top. Fine scratches show badly on gloss black. A domed cabochon hides them far better if the ring will be worn hard.
- Yellow gold is the strongest contrast and the traditional pairing; white metal reads more modern and more severe. Our metals guide covers both.
- Small diamonds work well against it. Black and white is the one colour combination that never argues, which is why the pairing is a permanent fixture.
- Buy the cut. With no colour or clarity to assess, the entire quality difference is in symmetry and polish, and it costs almost nothing to get right.
A short buying checklist
- Check the black is even, with no grey patches or ghost banding under strong light.
- Check the polish is glassy rather than satin, since a dull onyx is simply unfinished.
- Look at the surface for pits and undercut spots.
- Confirm it is chalcedony and not calcite, especially for anything carved or unusually large.
- For a cameo or intaglio, judge the carving, because that is what you are paying for.
- Do not pay a premium for untreated black onyx, which effectively does not exist.
Frequently asked questions
Is black onyx dyed? Effectively all of it is treated, by a sugar and acid process used since Roman times. It is permanent, penetrates the whole stone and is universally accepted rather than disclosed as a flaw.
Is onyx the same as black agate? They are the same material, chalcedony. The distinction is banding pattern: straight parallel bands are onyx, curved or concentric bands are agate. Solid black treated stone is sold as onyx.
Is onyx hard enough for a ring? Yes. At 6.5 to 7 with no cleavage it is practical for daily wear. The polish will scuff on a flat surface over years, and repolishing is cheap.
What is the difference between onyx and onyx marble? Onyx marble is banded calcite, a different and much softer material used for ornaments and tiles. It is not suitable for rings and should not be confused with the chalcedony.
Why is my onyx turning grey? Almost certainly surface scratching scattering light rather than the colour fading. Treated onyx does not fade. A repolish will bring it back.
Is onyx bad luck? Some traditions associate it with grief because of its use in mourning jewellery, and a few older sources call it a stone of discord. Others treat it as protective. There is no consistent tradition either way.
Can it go in an ultrasonic cleaner? Usually yes for a solid stone, but not if it is an assembled doublet or glued into its setting. Warm soapy water is always safe.
How do I clean it? Warm water, mild soap and a soft cloth. Dry it properly, since water spots show on gloss black more than on any other stone.
To see how it sits against the rest of the coloured stones, the semi-precious stones guide compares the series side by side, and we are happy to answer questions directly (opens in new tab).