Onyx in jewellery: from Roman seals to men's signet rings

Most of the ancient signet rings pulled from the ruins of Pompeii and Rome reached us intact. Not because of the black colour, and not because of any magic, but because of the stone itself: onyx is hard, dense and does not crumble under the carver's tool. So when a Roman magistrate ordered a gem for his seal, he almost always chose onyx. The impression in wax came out crisp, and the stone outlived its owner by a couple of thousand years.

Onyx is a variety of chalcedony, a cryptocrystalline form of quartz. Black, banded, cool to the touch, with a hardness of around 7 on the Mohs scale. That toughness and its handsome layering made it a favourite of gem cutters, Renaissance jewellers and the makers of Victorian mourning pieces. Today it is the stone of restrained men's signet rings, sealing rings and bracelets.

What follows is straight to the point: what onyx is made of, how it forms, where it is mined, how to tell it from fakes and look-alikes, how to care for it and what to wear it with. No mysticism, no promises that the stone will change your life.

What onyx is: composition, structure and properties

Chemistry and mineralogy

Onyx is chalcedony, the cryptocrystalline (microcrystalline) variety of quartz. Its chemical formula is silicon dioxide, SiO₂. What sets it apart from clear rock crystal is that it consists not of one large crystal but of countless microscopic quartz fibres packed tightly together. That is why the stone is opaque, or translucent only at the edges.

The defining feature of onyx is its parallel banding. Layers of different colour, most often black and white, were laid down one after another, so a cut section shows even, straight stripes. Its close cousin agate has concentric, curved bands, while onyx has straight, parallel ones. Those even layers are exactly what made onyx ideal for cameos and seals: the carver shaved away the pale top layer so a figure stood out against the dark ground.

The colour of the bands comes from trace impurities. Black and dark grey come from carbon and iron compounds, while the reddish-brown tones, as in sardonyx, come from iron oxides. Pure chalcedony on its own is colourless or milky.

One thing should be said up front: a large share of the black onyx on the market is grey or unremarkable chalcedony, dyed by the old sugar-and-acid method. This is a long-standing, accepted practice going back to antiquity, not a deception in itself. The problem only arises when a dyed stone is passed off as a rare, naturally black one.

Physical properties

• Mohs hardness: 6.5 to 7. Onyx scratches glass easily, but sapphire and diamond will scratch it. That hardness is enough for a piece to last decades without visible wear.
• Crystal system: trigonal (like all quartz), but at the level of individual microcrystals; to the eye the stone looks solid.
• Density: around 2.6 g/cm³. The natural stone is noticeably heavier than plastic (roughly 1.2 g/cm³), but lighter than, say, haematite.
• Lustre: vitreous to waxy; once polished, the surface is mirror-like.
• Fracture: conchoidal, as in all quartz.
• Optics: refractive index in the region of 1.53 to 1.54, weak birefringence. Because the stone is microcrystalline and opaque, it shows no dispersion (fire) and no marked pleochroism. This is not a faceted transparent gem but a dense ornamental material.
• Translucency: along thin edges natural chalcedony usually lets a little light through.

Because of the banding, onyx has weak planes of cleavage along the layer boundaries. In practice that means a stone with pronounced layers can split along a layer under a sharp blow or a sudden change in temperature.

How onyx forms: geology and deposits

Origin in nature

Onyx, like agate, forms in cavities within volcanic and sedimentary rock. Gas bubbles in cooled lava (amygdules) or cracks and hollows in limestone are filled over time by silica from circulating low-temperature solutions. The silica settles layer upon layer on the walls of the cavity. When conditions change periodically (the make-up of the solution, the impurities, the rate of deposition), bands of different colour form. Parallel layers give onyx, concentric ones give agate.

The process is slow and runs at low temperatures in near-surface conditions. The finished stone is chemically stable: it does not fade in the light and is not broken down by water.

Where it is mined

The main sources of onyx and agate material have been known for a long time:

• Brazil (especially the states of Minas Gerais and Rio Grande do Sul), the world's largest supplier of agate and chalcedony, including the material that becomes black onyx after treatment.
• Uruguay, high-quality agates and chalcedonies, historically used for carved work.
• India (the Deccan, the Gujarat region), one of the oldest centres for mining and working agate and carnelian; the tradition of stone carving here goes back centuries.
• Germany, Idar-Oberstein: the historic centre of agate cutting and treatment in Europe; this is where the methods of dyeing chalcedony were perfected.
• Madagascar, the United States (Oregon, Montana), Mexico, well-known sources of agates and chalcedonies in various shades.

Sardonyx with natural reddish-brown bands was historically mined and worked in India and Arabia; in Europe it was long prized as a valuable material for gems.

Onyx in culture and history

Antiquity: seals and gems

In ancient Greece and Rome, onyx and sardonyx were the main material for carved gems, for intaglios (an image cut into the stone, for sealing) and cameos (relief). A seal stood in for a signature: its impression in wax or clay authenticated a document. The contrast of the bands and the hardness of the stone made onyx ideal for the job, the fine design did not wear away through thousands of impressions.

The word "onyx" comes from the Greek onyx, meaning "nail" or "claw". Pale translucent bands over a dark ground reminded the ancients of the lighter crescent on a fingernail. The stone appears in classical authors among the gems used for carving; Pliny the Elder, among others, wrote about the properties and varieties of onyx in his "Natural History".

The Renaissance: cameos from layered stone

Carving in layered stone had its golden age in the Renaissance. Master cutters learned to shave the layers so that a pale figure rose above a dark ground, producing many-coloured cameos. One of the most famous examples of sardonyx carving, the "Gonzaga Cameo" (a Hellenistic work that later passed into European collections), shows what the technique was capable of: a double portrait carved using the natural layers of the stone.

The nineteenth century: mourning jewellery

In the nineteenth century black onyx took a firm place in what is called mourning jewellery. The Victorian age developed an elaborate etiquette of mourning, and dark stones, onyx, jet, black enamel, became the fitting choice for brooches, rings and pendants worn in memory of the dead. This is also when the pairing of black onyx with pearls and pale stones took hold: the contrast of dark and light read as restrained and severe.

In parallel, onyx remained the material of men's signet rings carved with monograms and coats of arms, a tradition inherited directly from the ancient gems.

Types of onyx and how it differs from similar stones

Varieties

• Black onyx, the most common jewellery type: uniformly black or with white bands. The bulk of the market is treated (dyed) chalcedony.
• Sardonyx, onyx with reddish-brown or brown bands instead of black, alternating with white. The red tone comes from iron oxides. It was historically valued more highly for its warm colour and its effect in cameos.
• White-grey and honey onyx, the pale banded varieties of chalcedony.
• "Onyx marble" (marble onyx), a separate story and a frequent source of confusion. It is not chalcedony at all but banded calcite or aragonite (calcium carbonate). It is used for vases, table tops and boxes. It is soft (hardness 3 to 4), scratches easily and reacts to acid. It has nothing to do with jewellery onyx-chalcedony, though it is sold under a similar name.

Onyx or agate

Onyx and agate are close relatives, both chalcedonies. The difference is in the pattern of the bands: agate has concentric, curved stripes following the shape of the cavity, while onyx has straight, parallel ones. In their properties (hardness, density) they are practically identical.

Onyx or black tourmaline, obsidian, jet

Black stones are easy to mix up, but their nature is different:

• Black tourmaline (schorl), a separate mineral, a silicate, harder than onyx (7 to 7.5), often with lengthwise striations on the faces and no banding.
• Obsidian, volcanic glass, amorphous, softer (around 5 to 5.5), with a characteristic glassy lustre and conchoidal fracture, sometimes translucent in smoky brown. Its nature is altogether different: there is more on this dark stone in the piece on obsidian in jewellery.
• Jet is not a stone in the mineralogical sense but a variety of fossil coal. Very light, warm to the touch, soft. It was jet, not onyx, that was the most popular "mourning" black material of the nineteenth century.

How to tell it from fakes and imitations

Dyed glass, plastic or heavily treated chalcedony is often sold as onyx. What helps at an everyday level:

• Weight and heat conduction. Natural stone feels cool to the hand and warms more slowly than plastic; glass feels lighter than stone of comparable size. This is a rough check, not proof.
• Translucency. The thin edges of natural chalcedony usually let a little light through. A completely opaque "onyx" may turn out to be glass or plastic.
• Bands under a loupe. In natural onyx the layers are imperfect, with tiny irregularities. A pattern that is too even, almost "printed", is a reason to be wary.
• Bubbles. Round air bubbles inside are a typical sign of glass, not stone.

Only a gemmologist with instruments can reliably tell natural colour from dye, let alone identify an imitation. Dyeing chalcedony is in itself a legitimate and ancient method of treatment; the only thing that matters is that the seller states it honestly.

How to choose onyx: what to look for in a finished piece

You don't buy onyx as a loose stone but as part of a ring, a signet, a pendant. So it is the whole piece that should be assessed, not the stone in mid-air. Here is what an experienced buyer's eye runs over.

Polish. The main indicator of quality work. Well-polished onyx gives an even mirror reflection with no matte patches and no "orange peel". Turn the stone under a lamp: the highlight should glide cleanly, without graininess. Cloudy areas mean under-polishing or micro-cracks beneath the surface.

Evenness of the black. In evenly dyed black onyx the colour is deep and free of brownish streaks. Greyish or brownish patches seen against the light point to uneven dyeing. This is not a fault in itself, but uneven colour is nothing to overpay for.

Chips along the edges. The vulnerable spot in onyx is the rim of the insert, especially with the flat plates used in signets. Run a fingernail along the edge of the stone and where it sits in the setting: catching chips and nicks are felt before they are seen. A good setting covers the rim with metal precisely to protect that edge.

Seating in the setting. The stone should not move in its bed. A slight wobble means the insert will fall out in time. For onyx a closed (bezel) setting or prongs that hold the stone firmly are preferable to a glued, butt-jointed seat.

Symmetry for beads. In a bracelet the beads should be of one calibre and free of flattened "barrels". Beads of varying size are a sign of cheap mixed grading.

A word on the honest question of dyeing. The overwhelming majority of black onyx is treated, and that is the norm. What should put you on guard is not the dyeing itself but an attempt to sell dyed chalcedony at the price of a rare, naturally black stone. A direct question to the seller, "natural colour or treated?", and a calm, specific answer, is already a good filter.

Insert shape: cabochon, plate, beads, intaglio

Onyx is not cut into facets like clear gems: it is opaque, it has no fire to play. So the stone is worked differently, and the shape has a direct bearing on how wearable it is.

A smooth cabochon (a domed polished drop with no facets) is the most common shape for pendants and women's rings. The domed surface reflects light in one soft highlight and stands up well to handling.

A flat plate goes into signets and men's rings. Its plus is the surface area for engraving, its minus is the sharp edges, which chip when knocked against a door frame or a table. A plate needs a setting that covers the rim.

Beads (4 to 8 mm for bracelets, smaller for strands) are convenient because the round shape has no vulnerable edges, so an onyx bracelet stands up to daily wear better than a ring with a flat insert.

Intaglio and cameo are carvings, cut in or in relief, the direct heir of the ancient gems. On solid black onyx an intaglio (a seal) is cut, on banded sardonyx a cameo is made using the contrast of the layers. Carved inserts are more sensitive to blows along the line of the cut, and should be worn more carefully than a smooth stone.

The practical takeaway: for a ring that will live an active life, a cabochon or a setting-protected plate is safer; for a dress piece you can allow yourself carving; for an everyday accessory worn without a thought for knocks, beads.

Myths about onyx

Caring for onyx

Onyx is hard enough (6.5 to 7) to resist scratches in everyday wear. Its weak points are the banding and a sensitivity to sudden changes in temperature, plus the fact that many stones on the market are dyed, and harsh chemistry can affect the dye.

Cleaning. Warm water, a drop of mild soap, a soft brush or cloth. Rinse, wipe dry. That is enough for regular care.

What to avoid. Ultrasonic and steam cleaning are inadvisable: vibration and heat can run along the layer boundaries or damage filled cracks. Don't keep the stone in direct sunlight or near sources of heat, or dip it in acids, chlorinated water and solvents. Put on perfume and cosmetics before you put on the jewellery, not after.

Storage. Apart from harder stones (diamond, sapphire and topaz will scratch onyx) and from anything it can scratch itself (pearls, silver). A soft pouch or a separate compartment of a jewellery box, in a dry place free of sharp temperature swings, suits it best.

Wearability. For rings and signets that take knocks, a hardness of 6.5 to 7 is a sensible minimum: onyx holds up better than soft stones like opal or jet, but it is still not eternal. Pendants and earrings wear less than rings and last longer.

What to wear onyx with

Onyx works not on its own but in the context of an outfit. A black stone draws exactly as much attention as the background gives it. The calmer the clothing, the more expressive the stone.

For everyday and for work, onyx is almost a sure thing. A grey or navy suit, a white or pale shirt, a silver signet with a black stone on the ring finger. The stone does not argue with a business look, it underlines a sense of composure. A slim bracelet of onyx beads under a shirt cuff also belongs in the office: barely seen, but the wrist feels the weight.

An evening out shifts the accents. A dark roll-neck, a shirt with the top button undone, the open collar of a polo. Here a pendant with onyx on a short chain works well, closer to the collarbone, so the stone reads against bare skin or dark fabric. Black on black looks restrained and expensive. For a special occasion add a single contrast: a silver or platinum metal next to onyx gives that very play of cool shine against matte black that was prized as far back as the nineteenth century.

The logic of combinations is simple. Onyx gets along with cool metals (silver, white gold, platinum) and with white, grey, navy and burgundy in clothing. It likes the company of white stones and pearls, where dark and light reinforce one another. Bright coloured stones next to onyx, on the other hand, compete for attention, and the stone loses. If you wear several pieces at once, keep them in one range: black onyx, silver, perhaps a single pale accent, no more.

Two practical tips. First: one striking onyx piece beats three small ones. The stone likes solitude. Second: for a signet take a massive stone and a cool metal, for a pendant a short chain, for a bracelet plain beads with no shiny inserts.

The symbolism of onyx: what to know, with a healthy scepticism

Different traditions credited onyx with properties of protection, grounding and resilience. The black colour was linked with seriousness and dignity, which is where the stone's role in mourning and ceremonial jewellery comes from. In the Indian astrological tradition onyx is associated with Saturn; in some medieval lapidaries it was given now protective, now gloomy properties, opinions diverged.

All of this is cultural symbolism, not proven properties. There is no confirmed effect of the stone on health, sleep, blood pressure, anxiety or luck. Onyx does not heal and does not "charge you with energy". If the stone appeals to you and supports a certain frame of mind, that is the ordinary psychological effect of a beautiful thing, nothing more. Buy a piece because it is beautiful and well made, not for the wonders it is promised to work.

Men's jewellery with onyx

The signet ring

The direct heir of the ancient gem. Today an onyx signet is worn as an ornament, sometimes engraved with initials or a monogram. The black stone looks restrained and does not shout the way yellow gold does. For a signet you take a massive stone (roughly from 12 by 16 mm) so it does not get lost, and a cool metal, sterling 925 silver gives an expressive contrast. Keep the engraving simple: one or two letters or a spare symbol read better on onyx than fine, wordy carving. On how to put together a monogram so the letters do not fight one another, see the guide to initials and monograms in jewellery.

The bracelet

Most often this is a bracelet of onyx beads (usually 4 to 8 mm) on an elastic cord or chain. It is spare, does not glint and suits daily wear. Another option is a silver bracelet with a single flat onyx insert.

The pendant

A polished onyx cabochon in a silver setting on a chain. Under a shirt and jacket it gets lost, but with an open collar or a dark roll-neck it reads well. A short chain holds the stone closer to the collarbone.

Onyx in women's jewellery

Black onyx is wrongly thought of as a purely men's stone, though historically it lived in women's jewellery no less. In the Victorian age it was women who wore onyx brooches and rings as part of the mourning wardrobe, and the classic pairing of black onyx with white pearls, which still reads as severe elegance, was born precisely in women's jewellery of the nineteenth century.

In a woman's look onyx works on contrast. It adds no colour, it adds graphic line: a sharp dark point or stroke against a pale ground. That is why pairings with white and clear are especially effective, onyx stud earrings with a pearl detail, a ring with a black cabochon next to a pale metal, a slim pendant on a long chain over a plain dress.

The logic of formats differs from the men's. Where men's jewellery takes onyx in a massive way (a signet, a large insert), women's pieces do better with small and medium forms: a 6 to 10 mm cabochon in a ring, drop earrings, a strand of small beads. A large black stone in a woman's ring sits well in a severe, almost business-like look and argues with a romantic one.

Black onyx is also versatile in tone: it pairs with both warm yellow gold and cool silver, because the stone itself has no colour temperature. That makes it a handy base when you would rather not match the stone to a particular metal.

FAQ: common questions about onyx

Is onyx a precious stone?

No, it is an ornamental (semi-precious) stone, a variety of chalcedony. It is more affordable than the classic precious stones, but it is beautiful and durable.

How does onyx differ from sardonyx?

In the colour of the bands. Onyx has black-and-white ones, sardonyx reddish-brown with white. In composition and hardness they are one and the same material, chalcedony.

What is the hardness of onyx?

6.5 to 7 on the Mohs scale. It scratches glass but is softer than sapphire and diamond. For everyday jewellery that is enough.

Can onyx crack?

Yes, under a hard blow or a sudden change of temperature, and the split most often runs along a layer boundary. With ordinary careful wear the stone lasts decades.

Is most black onyx dyed?

Yes, a large share of jewellery black onyx is treated (dyed) grey chalcedony. This is a long-standing and accepted method. The colour is stable and does no harm to the stone; the only thing that matters is that the seller does not hide it.

How do I tell onyx from black tourmaline and obsidian?

Tourmaline is a separate mineral, harder, often with lengthwise striations and no layers. Obsidian is volcanic glass, softer, with a glassy lustre and conchoidal fracture. Onyx usually shows parallel layers under a loupe.

Can I wear onyx in water, in the shower, the pool, the sea?

Water does the stone itself no harm, but chlorine and salt can affect the mount and the metal of the setting over time. Better to take the piece off before the pool and the sea.

What is the "onyx marble" in vases and table tops?

It is a different material, banded calcite or aragonite, soft (hardness 3 to 4). It does not belong to jewellery onyx-chalcedony, though the name is similar.

How do I clean onyx at home?

Warm water, mild soap, a soft brush or cloth. No ultrasound, no steam, no acids or solvents.

Is onyx suitable for women?

Yes. It is not a "men's only" stone: black onyx belongs in women's rings, earrings and pendants, especially in severe, contrasting looks with pearls or pale stones.

About Zevira

We make jewellery with onyx and sardonyx built for long wear: signets, sealing rings, bracelets and pendants in sterling 925 silver, with the option of engraving.

For us a stone is first of all a material with understandable properties, not a source of promises. We say honestly where the onyx is of natural colour and where it is treated chalcedony, we pick stones with an even polish and no cracks, and we set them in a mount that protects the vulnerable edges. We keep the engraving simple and legible, because that is how it looks best on onyx.

Open the catalogue of onyx jewellery

Back to home