Onyx is a captivating gemstone, recognized by its distinctive banded appearance, which has been admired and utilized across various cultures throughout history. It finds widespread application in decorative arts, jewelry, and ornamental objects. Onyx continues to be a popular choice for both artisans and collectors.
Chemical and Mineral Makeup
Onyx is primarily a variety of chalcedony, a microcrystalline form of quartz. Its chemical composition is silicon dioxide (SiO2). Onyx consists of tiny quartz crystals intergrown into a compact mass, too fine to be seen without magnification. This cryptocrystalline structure contributes to its smooth texture and often translucent quality. Impurities within this silicon dioxide structure can introduce varied colors and banding patterns.
Geological Formation
The formation of true onyx is a gradual process involving the deposition of silica-rich solutions within rock cavities. These solutions slowly evaporate, causing the dissolved silica to crystallize and form successive layers of chalcedony. This process often occurs in areas with volcanic activity or where silica-laden groundwater seeps into fissures and voids. The characteristic banding in onyx results from subtle changes in the composition or concentration of these silica-rich solutions during deposition.
Identifying Features
True onyx is distinguished by its characteristic parallel banding, where layers of varying colors run straight or nearly straight across the stone. This linear banding differentiates it from agate, another form of chalcedony, which typically exhibits concentric or curved bands. While often recognized for its classic black and white layers, onyx can also display a spectrum of other colors, including brown, red, green, and occasionally blue. Genuine onyx typically has a hardness of 6.5 to 7 on the Mohs scale, making it relatively durable.
Common Varieties and Misidentifications
One recognized variety of true onyx is sardonyx, which features white bands on a reddish-brown to orange base. A common point of confusion arises with “marble onyx” or “Mexican onyx,” which, despite the name, is chemically distinct from true onyx. This widely used material is a banded form of calcite, composed primarily of calcium carbonate (CaCO3), rather than silicon dioxide. Marble onyx is significantly softer, rating around 3 on the Mohs scale, and will react to acid, unlike the harder, acid-resistant true onyx.