Sardonyx

Sardonyx combines two kinds of chalcedony, sard and onyx: bands of brownish red to dark orange sard alternate with layers of onyx that are usually white or black. The contrasting layers have been prized for centuries for carving cameos and engraved seal stones. In the modern list of August birthstones it stands alongside peridot and spinel.

Cameos and engraved stones: the contrast between its colour layers has made sardonyx a favourite material for cameos and intaglios (engraved seal stones) for centuries; the figure is carved in one colour layer and the background in another. Jewellery: it is used as beads, cabochons and brooches. History: Roman soldiers wore sardonyx rings with Mars carved on them, and in the Renaissance it was preferred for cameos and decorative art.

It has a hardness of 6.5–7 on the Mohs scale. It scratches glass and steel; a steel knife cannot scratch it. Its specific gravity is 2.59–2.61 g/cm³.

Its best-known localities: Gujarat, India; Rio Grande do Sul, Brazil; Idar-Oberstein, Rhineland-Palatinate, Germany; Artigas, Uruguay; Madagascar.

It is made of straight, parallel bands: layers of brownish-red, orange or reddish sard alternate with white or black ones. Its hardness is 6.5–7: it scratches glass, and a steel knife cannot scratch it. The sard bands let light through their thin edges and look reddish; the stone has a waxy lustre, and a glassy shine when polished. Heated or dyed chalcedony: artificial treatments have been used since ancient times to produce the reds and yellows in sardonyx. The seller should disclose the treatment.

On the “street to diamond” value ladder it sits on step 5 of nine: Ornamental stone. The value of sardonyx depends on how crisp, contrasting and regular its bands are, how attractive its colours are, whether it is free of cracks and how well it is polished; in engraved cameos and seal stones, the quality of the carving decides. Because its hardness is only 6.5–7 it needs care in rings. Heated or dyed stones are common and worth less than naturally coloured ones, and the treatment should be disclosed.

Greek and Roman soldiers are said to have carried sardonyx talismans engraved with symbols such as Mars or with portraits, believing that the stone gave them strength, courage and protection in battle. In Renaissance Europe, wearing sardonyx was believed to give the speaker eloquence. According to a traditional Persian belief sardonyx helped against epilepsy; English midwives are said to have laid the stone between the mother’s breasts to ease childbirth. Today the stone is believed to encourage integrity, clear communication and a lifetime of happiness, and to stand for enduring relationships. There is no scientific evidence that sardonyx gives courage or protection in battle, strengthens speech, helps against epilepsy or eases childbirth. Epilepsy and childbirth need medical care; a stone is no substitute for treatment or for maternity care. This section reports beliefs; it is not health information. There is no scientific evidence for these claims and no stone replaces medical treatment.