Onyx is a banded variety of chalcedony. The colors of its bands may range from white to every color except purple or blue. Usually specimens of onyx contain bands of black and white.

Onyx comes through Latin from a Greek word meaning “claw” or “fingernail”. With its flesh tone color, onyx can be said to resemble a fingernail.

Onyx is formed of bands of chalcedony in alternating colors consisting of fine inter growths of quartz and moganite. It is cryptocrystalline. Its bands are parallel to one another when compared to the more chaotic banding seen in agates.

Sardonyx is a variant in which the colored bands are sard or shades of red rather than of black. Black onyx is perhaps the most famous variety, but it is not as abundant as onyx with colored bands.

Artificial treatments have been used since ancient times to produce both the black color in “black onyx” and the reds and yellows in sardonyx. Most of the “black onyx” on the market is artificially colored.

Banded calcite found in Mexico, Pakistan and other places are carved, polished and sold as onyx. But this is softer than true onyx and is more abundant. Artificial onyx types have also been produced from common chalcedony and plain agates.

Treatments for producing black and other colors included soaking chalcedony or boiling it in sugar solutions. It was treated with sulfuric or hydrochloric acid to carbonize sugars which had been absorbed into the top layers of the stone.

These techniques are still used in addition to the other dyeing treatments. So most so-called “black onyx” sold is artificially treated. In addition to dye treatments, heating and treating with nitric acid has been used to lighten or eliminate undesirable colors.