Onyx is a variety of chalcedony, a microcrystalline quartz, with straight, parallel bands of contrasting color. Classic onyx has black and white layers. Most solid black onyx in jewelry is chalcedony dyed black, a treatment used for well over a thousand years. Sardonyx, with red-brown and white layers, is one of the August birthstones.
Onyx and agate
Onyx and agate are both banded chalcedony and form in the same way, as silica-rich water fills cavities in rock and deposits layer after layer of tiny quartz crystals. The difference is the shape of the bands. Agate bands curve and follow the outline of the cavity. Onyx bands run flat and parallel. The two often occur in the same nodule, with curved agate bands at the edges and flat onyx layers that settled in the bottom.
The broader chalcedony family includes carnelian, chrysoprase and bloodstone, along with banded forms such as these. All share the same hardness and porosity.
Colors and named varieties
The trade uses several names for onyx by color:
- Black and white onyx is the classic form, with sharp layers of each.
- Sardonyx has layers of sard, a brownish red chalcedony, alternating with white or black.
- Carnelian onyx pairs orange-red carnelian with white layers.
- Black onyx usually means a solid black stone, nearly always dyed.
The name comes from the Greek word onyx, meaning fingernail or claw. Ancient writers used the word loosely for banded stones.
Why black onyx is dyed
Natural black chalcedony is rare. Most black onyx starts as gray or banded chalcedony. Chalcedony is slightly porous between its tiny crystals, so it absorbs liquids. In the classic method, the stone is soaked in a sugar solution and then heated in sulfuric acid. The acid carbonizes the sugar inside the pores and leaves a deep, even black. Pliny the Elder described stones boiled in honey, an early form of this method, and the process became an industry in Idar-Oberstein, Germany, in the 19th century.
The result is stable and permanent, and the trade accepts dyed black onyx as standard. Sellers should still disclose the treatment. Dyeing can also darken the porous layers of banded material and leave the dense layers white, which sharpens the contrast in black and white onyx. The gemstone treatments page covers dyeing alongside other common methods.
Sardonyx and the August birthstone
Sardonyx is one of the oldest birthstones on record. It appears on the traditional lists and remains an August birthstone alongside peridot and spinel. Its stacked red and white layers suited Roman signet rings and seals, and ancient writers claimed that sealing wax did not stick to it. The birthstones by month page lists the August stones.
Onyx in cameos and carving
Layered onyx is a classic material for cameos. A carver cuts the design into the upper light layer and removes the material around it to expose the darker layer beneath, so the figure stands out in white against a black or brown background. The reverse, an intaglio, is cut into the stone and was used for seals.
Ancient Greek and Roman engravers carved cameos and intaglios in sardonyx and onyx, and Renaissance workshops revived the art. The ancient Roman jewelry page describes signet rings and cameos in more detail. Victorian mourning jewelry also used black onyx widely, often beside jet.
Onyx is easy to carve into beads, inlay and flat plaques. Its uniform black makes it a common choice for men's signet rings, cufflinks and Art Deco pieces, where it was paired with diamonds and platinum.
Onyx marble is not onyx
Stone sold as onyx marble, Mexican onyx or Pakistani onyx is a banded calcite or aragonite. It is not chalcedony at all. It forms in caves and around springs, where calcium carbonate settles in layers, similar to travertine. It is often green, honey or white and is used for tabletops, carvings and tiles.
Onyx marble is far softer, about 3 on the Mohs scale, and reacts to acid. A drop of weak acid fizzes on calcite and does nothing to chalcedony, though this test leaves a mark and is best left to a lab. True onyx at 6.5 to 7 is much harder and cooler to the touch.
Telling black onyx from other black stones
Many black materials end up in jewelry, and they can look alike once polished. Hardness, weight and luster help separate them. The table gives typical values.
| Material | Mohs hardness | Specific gravity | Notes |
|---|---|---|---|
| Black onyx (chalcedony) | 6.5-7 | 2.55-2.70 | Even black, waxy to glassy polish |
| Jet | 2.5-4 | 1.30-1.35 | Very light, warm to the touch |
| Obsidian | 5-5.5 | 2.33-2.60 | Natural glass, may show bubbles |
| Black spinel | 8 | 3.58-3.61 | Bright, glassy luster |
| Hematite | 5.5-6.5 | 4.95-5.26 | Metallic luster, very heavy |
| Black glass | 5-6 | Varies | Molded edges, bubbles |
Jet feels strikingly light in the hand, and hematite feels heavy. Onyx sits in between. The page on specific gravity of gemstones explains how gemologists measure density.
Durability and care
Onyx is hard enough for most jewelry, though it can chip on hard surfaces. It has no cleavage and breaks with a conchoidal fracture. Clean it with warm soapy water and a soft cloth. Avoid harsh chemicals and long soaks, which can affect dyes in some stones. Ultrasonic cleaners are usually safe for solid onyx, but avoid them for stones with cracks or inlays. The guide to cleaning jewelry safely has more detail.
Frequently asked questions
Is black onyx natural?
The stone is natural chalcedony, but its black color is almost always produced by dyeing. Naturally black onyx exists but is rare.
What is the difference between onyx and agate?
Both are banded chalcedony. Agate has curved bands, and onyx has straight, parallel bands.
Is onyx the birthstone for August?
Sardonyx, a red and white banded onyx, is a traditional August birthstone. Peridot and spinel are the other August stones.
Is onyx marble real onyx?
No. Onyx marble is banded calcite or aragonite, a much softer stone that only looks like onyx.