Onyx is a popular gemstone prized for its smooth, dark appearance, making it a staple in jewelry and decorative objects. This stone is a form of chalcedony, a microcrystalline variety of quartz, which gives it high durability. Onyx is a type of silica, or silicon dioxide, characterized by its unique layered structure.
Geological Identity and Formation
Onyx is classified as a cryptocrystalline material, meaning its component crystals are only visible under high magnification. It is primarily composed of fine intergrowths of quartz and its monoclinic polymorph, moganite. Onyx forms through the precipitation of silica from water in the voids, fissures, and gas cavities of host rock.
This process involves silica-rich groundwater, often found in volcanic regions, depositing thin layers of mineral matter over extended periods. The defining characteristic of true onyx is the presence of straight, parallel bands of color. These uniform, linear bands differentiate it structurally from agate, which displays curved or concentric banding patterns. The resulting stone possesses a hardness of 6.5 to 7 on the Mohs scale, making it suitable for carving and polishing.
Primary Global Sourcing Locations
The raw material for onyx is extracted from deposits located across several continents. Significant sourcing locations include countries in South America known for substantial gemstone production. Brazil and Uruguay are notable producers, often supplying the classic banded material sought after for its clarity and contrast.
Other important sources for onyx extraction are found in North America, particularly in parts of the United States and Mexico. Additionally, India and Argentina are recognized for their historical and ongoing contributions to the supply of chalcedony and onyx varieties.
Understanding Black Onyx Coloration
While onyx naturally forms with alternating bands of color, the uniform, deep-black stone commonly sold as “black onyx” is rare in its naturally occurring state. When truly black onyx is found in nature, the solid color results from a layer of dark material being thick enough to cut a complete stone without showing other bands. However, most commercial pieces are produced by enhancing naturally lighter or gray chalcedony or banded onyx.
This enhancement is a permanent and stable treatment used since ancient Roman times to achieve the desired jet-black hue. The most common method involves soaking the porous chalcedony material in a sugar solution for a prolonged period. Following this, the stone is treated with a strong acid, such as sulfuric or hydrochloric acid. The acid reacts with the absorbed sugar inside the stone’s pores, carbonizing it to create a dense, black pigment that results in the stone’s characteristic uniform, deep-black appearance.