The common association of the gemstone onyx with a deep, uniform black color is a widespread misconception. Onyx is defined not by its color, but by its structure as a type of chalcedony, a microcrystalline form of quartz. While black onyx is the most famous commercial variety, the stone naturally occurs in a range of colors and is characterized by its distinctive layering.
The True Nature of Onyx
Onyx is a form of silica, specifically a banded variety of the mineral chalcedony, which is composed of fine intergrowths of quartz and moganite. Both onyx and agate are types of layered chalcedony, but they differ in the geometry of their banding. Agate typically features curved or concentric bands, whereas true onyx is defined by its straight, parallel, and consistent layers.
The formation process involves silica-rich fluids depositing layers in the cavities of volcanic rock over time. If these layers solidify as flat, parallel sheets, the result is onyx. The bands often alternate in color due to variations in mineral impurities and formation conditions. This structural definition—the parallel banding—is the mineralogical fingerprint of onyx, making color a secondary characteristic.
Varieties Beyond Black
The natural colors of onyx extend far beyond the popular black hue, often appearing as alternating bands of contrasting colors. Historically, the most valued variety was not black, but a reddish-brown banded form known as Sardonyx. Sardonyx consists of bands of the reddish-brown mineral sard alternating with white or black layers.
The name “onyx” comes from the Ancient Greek word for “claw” or “fingernail,” referencing the flesh-toned and white-banded varieties that resemble a human nail. Other natural variations include Nicolo, which has a very thin, translucent upper layer that allows a darker base color to show through as a blueish-grey. These multi-hued, banded forms represent the true spectrum of the stone.
Why Black Onyx Dominates the Market
The uniform, deep black onyx seen most often in modern jewelry is not the stone’s most common natural state. While pure black onyx does occur naturally, it is extremely rare, meaning the vast majority of solid black onyx available commercially has been treated. The stone’s porous nature allows it to be permanently dyed to achieve the desired uniform color.
This dyeing process has been used since ancient times, typically involving soaking grayish chalcedony or banded onyx in a sugar solution. The stone is then heated in a strong acid, such as sulfuric acid, which carbonizes the sugar within the pores, leaving behind stable carbon particles that produce a rich, consistent black color. Because this treatment is widespread and stable, “onyx” has become synonymous with “black onyx” in the public eye.