Sardonyx

Sardonyx has many sources. GIA notes India for sardonyx with good contrast between the colored layers, and also lists Brazil, Germany, Czechoslovakia, Madagascar, Uruguay and the United States, among other localities.

Sardonyx is defined by its parallel bands: red to brownish sard layered with white or black onyx. Without bands, the same red to orange chalcedony is sold as carnelian (red to orange-red) or sard (brownish). The IGS notes that almost any chalcedony can be turned red by heating because it contains finely dispersed iron compounds, and that dyeing is a common, usually stable enhancement that should be disclosed; GIA likewise notes that sardonyx is commonly dyed.

Sardonyx is 6.5 to 7 on the Mohs scale, so take care when wearing it, especially in rings. High heat, such as in jewelry manufacturing or repair, may affect the color of dyed sardonyx. Use ultrasonic and steam cleaners with caution; warm soapy water and a soft-bristle brush are the safest option.

GIA describes sardonyx as the most ancient August birthstone, with a history going back more than 4,000 years. In ancient times it was popular for Roman seals and signet rings because hot wax would not stick to it, and for millennia its colored bands have made it a favored material for cameos and intaglios.

According to GIA, sardonyx today is associated with courage, happiness and clear communication, and with stability in marriage and partnerships.

Sardonyx is believed to be one of the stones of the High Priest's breastplate described in the Old Testament, where it was said to represent the strength of spiritual life. Roman soldiers wore sardonyx rings carved with the image of Mars, believing they gave protection in battle.