Onyx is a banded variety of chalcedony — cryptocrystalline quartz — defined classically by its straight, parallel bands of contrasting colour, most often black and white. The term has, however, been stretched in modern commercial use to denote any solid black chalcedony, regardless of whether bands are present, and the bulk of jewellery sold today as 'black onyx' is dyed solid chalcedony rather than naturally banded material. The name derives from the Greek onyx, meaning fingernail or claw, a reference to the translucent layered structure of the classical material as much as to its colour. Onyx has been carved for cameos and intaglios since antiquity, and the banded variety remains the technical material of choice for skilled cameo cutters who exploit the differential colour of the layers to create raised relief.

Mineralogy and physical properties

Onyx is composed of microcrystalline quartz with a hardness of 6.5 to 7 on the Mohs scale, a specific gravity of approximately 2.58 to 2.64, and a refractive index around 1.535 to 1.539 (single, with a low birefringence). The structure is fibrous-microcrystalline, like all chalcedony, and the optical character is technically uniaxial-positive in the constituent quartz fibres but appears effectively isotropic at hand-specimen scale. Cleavage is absent, fracture is conchoidal to splintery, and the material takes an excellent polish.

The defining feature of true onyx is the parallel banding. This distinguishes it from agate, which displays curved, concentric, or fortification banding, even though both are formed from the same chalcedonic chemistry. The bands in onyx are typically the result of layered deposition in cavities under stable conditions; the alternation of dense and porous bands accepts dye differently and is the basis of much of the dye treatment described below.

Dye treatment and the modern 'black onyx' trade

The vast majority of jet-black onyx in the modern market is treated chalcedony rather than naturally black material. The conventional dye process, used since at least the early nineteenth century, involves soaking porous chalcedony in a sugar solution and then treating it with sulphuric acid; the carbon residue left in the porous bands turns them permanently black. Sometimes the entire piece of chalcedony is rendered black; sometimes only certain bands take the dye, leaving a banded contrast suitable for cameo work. The treatment is stable, considered standard trade practice, and is rarely separately disclosed at retail because it is so universal.

Naturally black, naturally banded onyx of cameo quality is now uncommon enough that material sold under that description should be treated with scepticism unless backed by reliable provenance. The cameo-cutting trade in Torre del Greco, Italy, and in Idar-Oberstein, Germany, draws principally on dyed material today, and the technical quality of the cameo derives from the cutter's skill in choosing and orienting the rough rather than from the rarity of the natural banding.

Cameo and intaglio tradition

Onyx has been the classical material for cameo carving since Hellenistic times. The standard cameo technique exploits the layered structure: the cutter carves the upper band into a relief portrait or scene while leaving the contrasting lower band as a smooth ground. The most celebrated antique cameos — the Gemma Augustea in Vienna, the Great Cameo of France in Paris, the Cup of the Ptolemies in Paris — exploit multi-layered banded onyx and sardonyx with extraordinary technical skill. The Renaissance and the eighteenth-century neoclassical revival each renewed the cameo tradition, and the nineteenth-century Italian shell-cameo industry adapted the same compositional grammar to the cheaper substrate of conch and helmet shell.

Intaglio carving — incised rather than relief — is the older sister technique and was the basis of the classical signet ring and seal stone. Onyx and sardonyx held the bulk of the Roman intaglio production, and the carving tradition continued through the medieval and Renaissance periods.

Sources

The major sources of chalcedony suitable for onyx production today are Brazil, Uruguay, India, Madagascar, and Botswana. The Brazilian and Uruguayan deposits are by far the most important commercially, supplying the porous grey-white agate that is the standard substrate for dye to black onyx. India contributes additional rough, particularly from the western state of Gujarat. The decorative-stone trade also markets a range of so-called 'onyx' that is in fact banded calcite or aragonite — a separate material covered under the entry for onyx marble.

Identification

Distinguishing onyx from glass, plastic, or other black materials is straightforward in most cases. The hardness of 6.5 to 7 means a steel point will not scratch true onyx; specific gravity around 2.6 separates it from heavier glass and lighter plastic. Under magnification, onyx shows a microcrystalline texture, sometimes with visible bands, and may show characteristic dye-saturation patterns at the edges of porous areas. Fluorescence is generally inert. Heat treatment is rare and not commercially significant.

The most common identification challenge is distinguishing onyx from black tourmaline (schorl), black spinel, and black diamond. Hardness, density, and optical properties separate all of these readily; refractive index measurement and a quick density check are usually decisive.

In the trade

Black onyx remains a workhorse material in commercial jewellery. It is inexpensive, durable enough for any normal jewellery use, and takes a high polish that suits both Art Deco geometric designs and Victorian mourning jewellery. Cabochons, beads, inlays, and cameos are all common applications. Cleansing should be by mild soap and warm water; ultrasonic cleaning is generally safe but should be avoided for cameos with delicate undercutting or for material of unknown provenance. The dye treatment is stable, but onyx should not be exposed to strong acids or prolonged heat.

For cameo or intaglio work of artistic ambition, sardonyx — the brown-and-white banded variety — and the multi-layered onyx of classical sources remain the materials of choice. See also sardonyx, chalcedony, and onyx marble for related entries.

Further reading

• GIA — chalcedony reference
• GIA Gems & Gemology — chalcedony and dye-treatment articles
• International Gem Society — onyx and chalcedony
• GIA — quartz family overview