Sardonyx is a banded variety of chalcedony (cryptocrystalline quartz), with layers alternating between Sard and Onyx. It is an impure fine-grained quartz whose crystals are too small to see.
The color is usually a yellow-brown alternating with white, but other variants are also called sardonyx.
The banded nature of sardonyx is used to advantage by lapidaries and artisans, as a careful selection of material and design allows the creation of bas-relief art, where one layer is left as a foreground, and another is used as the background for a carving.
Sardonyx is the traditional birthstone for August. It is associated with relaxation and security.
Sardonyx is only one of several chalcedony (cryptocrystalline quartz) varieties. The primary varieties are as follows: