Onyx is a type of agate known as chalcedony, formed by intergrowths of silica minerals. The chert form of quartz is also used to make onyx jewelry items.
Natural onyx is used primarily as the gemstone in cameos. It is a flat variety of chalcedony with alternating bands that are usually black and white color. However, some onyx contains bands of red and white or brown and white. These other types of chalcedony are often called sardonyx because the bands are sard, or red-shaded, instead of black. Sardonyx is more common than black onyx in natural settings. However, much of the modern jewelry containing onyx or sardonyx gemstones are artificially colored. Various methods of coloration, such as through the use of dyes or sugar-sulfuric acid treatments, are used to create the black in onyx and the other colors in sardonyx.
The chert form of quartz crystals, which is a subset of quartz that contains crystal structures too small to be seen with the human eye, are also used as gemstones. Chert is formed in a variety of colors and can be found in jewelry such as tiger's eye, jasper, citrine, amethyst and onyx gemstones. Quartz is a very hard silicate and is used for onyx jewelry items that require more handling and wear.