Onyx

Onyx is a form of chalcedony, meaning it is quartz (SiO₂) built from microscopic fibrous crystals. What sets onyx apart from agate is the geometry of its banding: onyx shows straight, flat, parallel layers rather than the concentric or curved bands of agate. The classic black-and-white onyx, and the red-brown-and-white variety called sardonyx, have been carved into cameos and intaglios since antiquity.

Identifying onyx

True onyx is hard (about 7 on the Mohs scale, like all quartz), takes a high polish, and shows clean straight banding. It should not be confused with the soft, translucent banded calcite/aragonite sold as 'onyx marble' or 'Mexican onyx', which is a carbonate (hardness 3) and a completely different material despite the shared trade name.

Where it is found

Chalcedony and its banded varieties form in cavities and fractures in many rock types worldwide. Brazil, India, Uruguay and the United States are major sources, and much commercial black onyx is colour-enhanced agate. China is a long-established producer of agate and chalcedony, used both for specimens and for the carving and bead trades.

What collectors look for

Collectors and lapidaries look for crisp, high-contrast banding, even colour, and freedom from fractures. Natural untreated banded onyx, and fine sardonyx with clean red-and-white layers, are the most desirable.