Onyx is a widely utilized material, popular both as a durable gemstone in jewelry and as an elegant decorative stone in architecture. While many people associate the name with a single, deep black shade, the material’s actual color spectrum is far more diverse. The stone’s true colors range from natural shades of brown and red to white and gray. The commercial market introduces other colors through specialized treatments or the use of a similar, though chemically distinct, material. Understanding the colors of onyx requires distinguishing between the mineralogical definition and the common commercial usage of the name.
The Mineralogical Identity of Onyx
Mineralogically, true onyx is a banded variety of chalcedony, a microcrystalline form of the mineral quartz (silicon dioxide). The defining feature separating onyx from other chalcedony varieties is the structure of its layers. Onyx exhibits straight, parallel bands, unlike agate layers which are typically curved. This parallel structure is valuable for carving cameos and intaglios, utilizing the contrast between layers.
The stone forms as silica-rich groundwater deposits rhythmic layers inside cracks and voids, often in volcanic regions. True onyx possesses a hardness of 6.5 to 7 on the Mohs scale, making it a durable material for fine jewelry. Its color is inherently tied to the presence of trace elements and impurities within the layers.
The Natural Color Spectrum of Chalcedony Onyx
The natural coloration of chalcedony onyx is primarily defined by two variations: alternating bands of black and white, or bands of brown/red and white. The bands are typically translucent to opaque, with lighter layers consisting of nearly pure white or gray chalcedony. The darkness in the alternating layers results from high concentrations of carbon or iron oxides trapped during formation.
While “black onyx” is the most recognized term, naturally occurring, uniformly deep black onyx is quite rare. The classic variety features layers of black or dark gray alternating with white bands. The most common natural color variant is sardonyx, defined by reddish-brown, brown, or deep red layers (called “sard”) alternating with white or black chalcedony layers. This reddish-brown hue is caused by higher concentrations of iron oxide impurities.
Commercial Colors, Dyeing, and Common Misnomers
The wide array of colors seen in the commercial market, including jet black, vibrant greens, and blues, are often the result of human intervention. Most jewelry-grade deep black onyx sold today is produced by treating naturally lighter-colored chalcedony or agate. The traditional method involves soaking the porous stone in a sugar solution for several weeks, followed by immersion in a heated acid bath. This process carbonizes the absorbed sugar, converting it into black carbon that is permanently deposited deep within the stone’s microscopic pores. Other vibrant colors like deep blue or green are achieved through similar chemical dyeing processes using modern colorants.
Onyx Marble Misnomer
The consumer confusion regarding onyx color is amplified by the term “onyx marble,” which describes a completely different material. Architectural slabs sold as Green Onyx, White Onyx, or Blue Onyx are actually banded calcite or aragonite, a calcium carbonate material chemically distinct from true chalcedony onyx.
This calcite material, sometimes called Mexican Onyx or Oriental Onyx, is much softer, registering only 3 to 4 on the Mohs scale. It is often translucent and exhibits beautiful, colorful banding due to mineral impurities like iron oxide and copper, making it highly desirable for decorative applications such as backlit panels. If a stone is brightly colored, translucent, and used in large architectural pieces, it is almost certainly this softer banded calcite, and not the harder, mineralogically defined chalcedony onyx.