Gemstone Encyclopedia / Onyx
Onyx has been carved into seals, signet rings, and cameos since antiquity - a banded chalcedony prized for the sharp contrast between its layers. The deep, uniform black most people picture today, though, almost never occurs that way in nature: it's the result of a dyeing technique that's been in continuous use for roughly two centuries.
Overview
Nearly all the black onyx on the market today started out a different color entirely.
Onyx is a variety of chalcedony - the same microcrystalline quartz family as agate, carnelian, and chrysoprase - defined by straight, parallel bands of contrasting color. That's the detail that actually separates it from agate: agate's bands curve and swirl, while onyx's run in flat, even layers. Mineralogically, the two are close cousins, but the trade has always treated them as distinct stones.
Genuinely natural, solid black onyx is uncommon - most chalcedony that forms with real black banding has layers too thin to cut as a single solid color. The deep, uniform black associated with "onyx" today is almost always the result of dyeing: porous gray-white chalcedony is soaked in a sugar solution, then treated with hot sulfuric acid, which carbonizes the sugar inside the stone's pores and leaves it permanently black. It's not a new or deceptive trick - the technique has been documented since at least the early 1800s, and it's so standard in the trade that most gemological labs describe black onyx as "probably dyed" by default.
Like other chalcedony, onyx typically forms in the gas cavities of volcanic rock, where silica-rich groundwater deposits mineral layers over time, building up the thin bands that give banded chalcedony its name. It can occur in substantial masses - sometimes several pounds in a single piece - which is part of why onyx has stayed so affordable even as a gem with millennia of history behind it.
Colors
Solid black gets the name recognition, but onyx's banded relatives - each with their own name - are where the stone's real history lives. Drag the bar or pick a shade to compare.
A deep, uniform black with a glossy polish - the color most people picture when they hear “onyx,” and, in the large majority of cases, a dyed color rather than a natural one.
A banded variety with white or cream layers contrasting against reddish-brown ones - the traditional August birthstone, and a favorite material for Roman and Renaissance cameo carvers.
A banded combination of carnelian's warm orange-red with a contrasting white layer - not to be confused with plain carnelian, which is solid red-orange chalcedony with no banding at all.
A black base topped with a very thin, pale bluish-gray layer - thin enough that light passing through it gives the surface a faint blue cast, a favorite for Roman intaglio seals.
Colors shown are illustrations. Real onyx varies with individual stone quality, and the vast majority of solid black material is dyed rather than naturally occurring.
Properties
Mohs measures scratch resistance, from talc (1) to diamond (10).
At 6.5 to 7 on the Mohs scale with no cleavage, onyx resists chipping and scratching well and is considered durable enough for everyday jewelry - rings included.
The main thing to watch for isn't physical durability, it's the dye: prolonged exposure to strong, direct sunlight can fade dyed onyx over time, and its color only penetrates a few millimeters into the stone, so heavy repolishing can wear through it.
With onyx, treatment isn't a minor footnote - it's how almost every solid black stone on the market got its color in the first place. Select a level to see how the market actually breaks down.
The 4Cs
Solid black gets most of the attention, but onyx and its close relatives come in genuine banded varieties too - each with its own name and history. Open each of the four Cs to see how they fit together, and see the named varieties in the Colors section above.
For solid black onyx, color is about depth and evenness - a rich, uniform black with no visible gray undertone or patchiness is the goal, and dye quality varies noticeably between suppliers. For the banded varieties, it's the opposite: sharp, clean contrast between the layers is what carvers and collectors look for.
Because the color in most black onyx comes from dye rather than from the stone's own chemistry, "color grading" here is really about how well and how evenly the treatment was done, not about a naturally occurring hue range the way it is for most other gems.
See every named variety, from solid black to banded sardonyx, in the Colors section above.
Because onyx forms in large masses and is relatively abundant, it's typically sold by size (in millimeters, for cabochons and beads) rather than priced steeply by carat weight the way rarer faceted gems are. Large cabochons and statement pieces are genuinely affordable compared to almost any other gem of similar size.
Antique and finely carved cameos are the exception - there, the carving itself, not the raw size of the stone, is what drives value.
Onyx is opaque, so clarity in the diamond or emerald sense doesn't apply - there's no internal transparency to grade. What matters instead is surface quality: a smooth, glossy, even polish free of pits, scratches, or patchy, uneven dye color.
For cameos and intaglios, clarity-equivalent quality is about the carving itself - crisp, well-defined relief or incised detail, and clean contrast between the dyed and undyed layers being carved through.
Most onyx is cut en cabochon - a smooth, polished dome with no facets, since the stone is opaque and faceting adds little visual benefit. Oval and rectangular cabochons are both common, alongside freeform shapes that follow the rough material's natural outline.
The historic cutting style, though, is relief and incised carving: cameos (a raised design carved from the upper layer, with the layer below left as a plain background) and intaglios (the reverse - a design cut into the stone, the basis of classical signet rings and wax-seal stones). Both rely entirely on a carver's skill at orienting the rough to get clean contrast between bands.
Origins & Value
Most of the chalcedony that becomes black onyx comes from a handful of sources - what varies by origin is less the final color, since nearly all of it is dyed, and more the quality of the raw material itself.
Origin
Brazil is one of the two most important sources of the porous, grayish chalcedony that becomes commercial black onyx once dyed. Its deposits are large and consistent enough to support steady supply at an accessible price.
Origin
Uruguay sits alongside Brazil as the other dominant commercial source of raw chalcedony for black onyx. Material from both countries supplies the great majority of dyed onyx in the market today.
Origin
India, particularly Gujarat, has long been a center for cutting and processing chalcedony into finished onyx cabochons, beads, and carved pieces, in addition to supplying some of its own raw material.
Origin
Madagascar has become an increasingly significant source of chalcedony as mining there has expanded, with Botswana contributing additional material. Neither rivals Brazil or Uruguay in volume yet, but both add meaningfully to overall supply.
Because color in most onyx comes from treatment rather than origin, where a stone was mined matters far less to its final appearance than it does for gems like sapphire or emerald - origin here is mostly a question of raw material quality and supply, not a distinct look.
Onyx is one of the more affordable gemstones overall - it forms in large masses, and the dyeing process that produces most commercial black onyx is inexpensive and well established. That affordability is part of its appeal, especially for bold, large-stone designs that would cost far more in a rarer gem.
The flip side is that onyx's recognition as "the black gemstone" makes it a frequent target for misrepresentation - black glass, plastic, dyed howlite, and other simulants are all sold as onyx at times, and vintage or antique pieces may carry treatments that were never disclosed. A simple scratch test (a steel point won't mark genuine onyx) and, for anything valuable, professional testing are the most reliable ways to confirm what you actually have.
Genuinely natural, undyed black material is the real rarity here - confirmed-natural stones are a specialty item precisely because so little chalcedony forms that way on its own.
Care
At 6.5 to 7 on the Mohs scale with good toughness, onyx holds up well to daily wear - the main thing to manage is its dye, not its durability.
A soft brush, warm water, and a drop of mild soap is the safest way to clean onyx; rinse and pat dry.
Mechanical cleaners can damage onyx, and are especially risky for cameos with delicate carved undercutting.
Dyed onyx - the large majority on the market - can fade with extended direct sun exposure.
Onyx has no particular heat sensitivity in normal wear, unlike some gems.
Keep onyx in its own pouch or a lined compartment to avoid scratching softer stones or jewelry.
Older onyx jewelry may carry treatments or repairs that weren't disclosed at the time - treat it with extra care.
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At Forever Rox Fine Jewelry, we select onyx for clean, even color and a flawless polish - the qualities that separate a well-made piece from an ordinary one. Whether it's a deep black cabochon in a classic setting or a banded sardonyx with real character, we're glad to help you find a piece with real presence. Browse onyx rings, earrings, necklaces, and custom-designed pieces.
FAQ
Almost always dyed. Genuinely natural, solid black onyx is uncommon - most black onyx on the market is porous chalcedony treated with a sugar-and-sulfuric-acid process that permanently carbonizes it black. The technique has been in standard use for roughly two centuries.
Both are banded chalcedony, but onyx has straight, parallel bands, while agate's bands curve and swirl. They're mineralogically close cousins, distinguished mainly by that banding pattern.
Sardonyx is a specific banded variety of onyx - white layered with reddish-brown, rather than onyx's classic black and white. It's also the traditional August birthstone, alongside peridot and spinel.
No. “Onyx marble,” sometimes called Mexican onyx, is banded calcite - a completely different, much softer mineral (around 3 on the Mohs scale) often used in home decor. True onyx is chalcedony, a form of quartz, at 6.5 to 7 Mohs.
Yes. At 6.5 to 7 on the Mohs scale with no cleavage, onyx resists chipping and scratching well. The main thing to avoid is prolonged strong sunlight, which can fade dyed color over time.
A steel point won't scratch genuine onyx, which rules out softer imitations like glass or dyed howlite. For anything valuable, professional gemological testing is the most reliable way to confirm it.
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