Onyx

Onyx is a banded form of chalcedony, a cryptocrystalline variety of quartz (SiO2). The defining feature is parallel, layered color bands, most often black and white. True onyx refers to the black-and-white form; when the bands are red or brown this variety is called sardonyx. Both are varieties of chalcedony.

Onyx forms in voids within volcanic rocks or in sedimentary environments where silica-rich fluids deposit successive layers of microcrystalline quartz. It occurs worldwide, with notable sources in Brazil, Madagascar, India, Mexico, and the United States (notably Arizona and Oregon).

In hardness terms, onyx has a Mohs hardness around 6.5–7 and a conchoidal fracture. It is typically translucent to opaque with a waxy to dull luster. Because of its attractive banding, it is widely used for jewelry, cameos, intaglios, and decorative carvings. The material can be cut en cabochon or carved into cameos and reliefs; it is commonly seen in agate-like slabs and beads.

Market samples are often treated or dyed to deepen the color or to create uniform black onyx. Synthetic chalcedony can resemble natural onyx. Because of silica content, working onyx generates silica dust; standard safety precautions apply.

Etymology: the name derives from Greek onux, meaning nail or claw, in reference to the color bands resembling fingernails.