The deep, uniform black of commercial "black onyx" is, in the overwhelming majority of cases, not a product of nature but of chemistry. Most material sold under this name is grey or banded chalcedony — a microcrystalline variety of quartz — that has been subjected to a well-established treatment known as the sugar-acid process, or sugar-soaked treatment, which deposits finely divided carbon particles throughout the stone's porous silica structure. The result is a stable, opaque, intensely black material that has supplied the jewellery trade for well over a century. Understanding this treatment is essential for gemmologists, jewellers, and informed buyers alike, not least because disclosure is mandated under the guidelines of the American Gem Trade Association (AGTA) and the United States Federal Trade Commission (FTC).

Natural Black Chalcedony versus Treated Material

True black onyx — chalcedony with naturally occurring black colouration — does exist, but it is genuinely rare. Onyx is technically defined as a parallel-banded variety of chalcedony, with alternating black and white layers; the term has, however, been used loosely in the trade for centuries to describe any opaque black chalcedony. Natural black colouration in chalcedony arises from included carbonaceous matter or from fine-grained iron oxide minerals distributed through the silica matrix. Such material is uncommon enough that it cannot supply global demand at commercial scale. The vast majority of chalcedony mined for the black onyx trade — much of it originating from Brazil, Uruguay, and India — is naturally grey, pale, or banded, and requires treatment to achieve the uniform black colour the market expects.

The Sugar-Acid Treatment: Mechanism and History

The sugar-acid process exploits the naturally porous microstructure of chalcedony. Because microcrystalline quartz contains a network of submicroscopic channels and pores, liquids can penetrate the stone to a meaningful depth when sufficient time and, in some protocols, gentle heat are applied.

The treatment proceeds in two principal stages:

• Impregnation: The rough or pre-shaped stone is immersed in a concentrated sugar solution — historically cane sugar, though other reducing sugars have been used — and allowed to soak for an extended period, sometimes days, at elevated temperatures. The sugar solution permeates the porous structure of the chalcedony.
• Carbonisation: The sugar-saturated stone is then immersed in concentrated sulphuric acid. The acid dehydrates and chars the sugar molecules within the stone's pores, depositing amorphous carbon — effectively fine soot — throughout the interior. This carbon is what produces the intense black colour.

The process is not a modern invention. Historical accounts and gemmological literature document its use in Germany's Idar-Oberstein cutting district from at least the mid-nineteenth century, where it became the industrial standard for producing black onyx from Brazilian chalcedony. Idar-Oberstein's craftsmen refined the technique over generations, and the treatment spread globally as the trade in cut and polished chalcedony expanded.

A variant approach uses other carbonisable organic substances in place of sugar, but the underlying chemistry — acid-catalysed carbonisation within the stone's pore network — is the same. Some sources also refer to iron-salt treatments used to produce brown or reddish tones in chalcedony by a related impregnation-and-oxidation mechanism, but for black onyx specifically, the sugar-acid route remains dominant.

Stability and Durability of the Treatment

The carbon deposited by the sugar-acid process is chemically inert and physically lodged within the silica matrix rather than merely coating the surface. Under normal conditions of wear — including exposure to light, perspiration, mild cleaning agents, and routine polishing — the colour is considered permanent and stable. Unlike some surface coatings or resin-based colour enhancements, the treatment does not fade, peel, or wash away. This stability is one reason the process has remained the industry standard: the treated material behaves, from a durability standpoint, much as a naturally coloured stone would.

Caution is warranted, however, with ultrasonic cleaning and steam cleaning. The same porosity that allows the treatment to penetrate the stone can, under aggressive cleaning conditions, admit chemicals or moisture that may — over time and with repeated exposure — affect the treated material. Standard jewellery-trade guidance recommends warm soapy water and a soft brush for cleaning black onyx pieces.

Gemmological Detection

Identifying the sugar-acid treatment requires a combination of observation and, where necessary, spectroscopic analysis. Several indicators are accessible to a trained gemmologist:

• Colour uniformity: Treated material typically displays a remarkably even, flat black throughout. Natural black chalcedony may show subtle colour zoning or variations under strong transmitted or reflected light.
• Surface and fracture examination: Under magnification, treated stones may show carbon concentrations along fractures or grain boundaries, sometimes appearing slightly more intense at these sites. A freshly broken surface — rarely available in finished goods — would reveal carbon deposition patterns consistent with pore-filling.
• Spectroscopy: Reflectance spectroscopy and, in some cases, Raman spectroscopy can detect the amorphous carbon signature within the stone. GIA's gemological laboratory and other major laboratories routinely identify this treatment through such methods. The carbon produces a characteristic broad absorption that differs from the spectral signature of naturally black chalcedony.
• Refractive index and specific gravity: These standard constants remain consistent with untreated chalcedony (RI approximately 1.53–1.54; SG approximately 2.58–2.64), as the treatment does not substantially alter the quartz matrix itself.

In practice, the trade assumption — supported by gemmological consensus — is that virtually all commercial black onyx has been treated. A stone represented as naturally black chalcedony without treatment would require laboratory verification to substantiate that claim.

Trade Usage, Disclosure, and Nomenclature

The AGTA's treatment disclosure codes require that dyed or treated stones be identified as such at the point of sale. The FTC's Guides for the Jewelry, Precious Metals, and Pewter Industries similarly require disclosure of treatments that affect value. For black onyx, the standard trade practice is to disclose the treatment, though the ubiquity of the treatment has led to a degree of normalisation: "black onyx" is widely understood in the trade to mean treated chalcedony, much as "blue topaz" implies irradiation and heat treatment.

The term onyx itself carries a complicated history. Strictly, onyx is banded chalcedony; the solid black material produced by the sugar-acid treatment is more accurately described as dyed black chalcedony. Nevertheless, "black onyx" is deeply entrenched in commercial usage and is unlikely to be displaced. Laboratory reports from GIA and equivalent bodies will typically note the material as chalcedony with an indication of treatment.

Pricing reflects the treated nature of the material. Black onyx is an affordable, widely available commercial stone; its value lies in its appearance, workability, and long association with mourning jewellery, men's accessories, and graphic black-and-white jewellery design rather than in rarity. Fine antique pieces set with black onyx — Art Deco jewellery in particular, where the stone's flat, geometric black surface was prized — may carry significant value as objects of craft and design, but this value is attributable to the jewellery itself rather than to the stone as a rare gem material.

Historical and Cultural Context

Black onyx has been used in jewellery and decorative objects since antiquity, though the material available to ancient craftsmen was almost certainly naturally coloured or dyed by cruder means. The Roman writer Pliny the Elder described onyx and its uses in the first century CE. The stone's association with mourning and funerary jewellery became particularly pronounced in Victorian England, where jet, black glass (French jet), and black onyx were all employed in mourning dress following the death of Prince Albert in 1861. The sugar-acid treatment, industrialised in Idar-Oberstein during roughly the same era, made black onyx available at a scale that could meet this demand.

In the twentieth century, black onyx became a staple of Art Deco jewellery, prized by Cartier, Van Cleef & Arpels, and other leading maisons for its graphic contrast against platinum, diamonds, and coral. These associations have sustained its popularity through to the present day.

Further Reading

• GIA Gems & Gemology — Gem Trade Lab Notes on Dyed Chalcedony
• AGTA Treatment Disclosure Codes
• GIA on Gemstone Treatments and Disclosure