Traditionally identified as Virgo's birthstone, the sardonyx promotes clear expression, healthy marital relationships and a positive state of mind. Because the sardonyx is a fire stone - ruled by the planet of Mars and associated with the warrior god of the same name - the assignment of this birthstone to Virgo raises problems. Virgo is an earth sign ruled by Mercury and associated with Mercury the god of communication.
To find a correspondence between this sign and the sardonyx requires an interpritive strength that would offend the integrity of those born under the sign of the virgin!! The varieties of agate associated with the element of earth provide a truer match for Virgo. Black and white agate, green agate and moss agate are associated with the Virgo's planetary ruler, airy Mercury, but they bear earthy energies that protect the wearers health, and we all know that protection of health is a distinctly Virgonian function.
Green agate emits energies that purify the eyes - and clear eyesight satisfies Virgo's need for truthful perception of earthly realities. The energy of moss agate promotes abundant garden growth evocative of Virgo's association with Ceres the roman goddess of grain and harvest.