σαρδόνυξ

Sardonyx is a compound: sárdion (a reddish-brown stone, probably named after the city of Sardis in Lydia) and ónyx (claw, nail — the Greek word for the banded variety of chalcedony). The stone has alternating layers of reddish-brown sard and white chalcedony, and these parallel bands are what made it irresistible to ancient gem carvers. A skilled glyptographer could carve a figure in the white layer against the red background, creating a natural two-tone cameo.

Roman emperors commissioned sardonyx cameos of extraordinary ambition. The Gemma Augustea, carved around 10 CE, is the size of a dinner plate and depicts Augustus receiving the submission of barbarians in the white layer over a dark background. The Great Cameo of France, the largest surviving ancient cameo, shows five generations of the Julio-Claudian dynasty. These were propaganda instruments — portable monuments carved into stone.

The August birthstone in many classical traditions, sardonyx was believed to bring courage and eloquence. Roman soldiers wore sardonyx signets into battle. The stone was relatively affordable — never as precious as emerald or sapphire — which made it the gemstone of the ambitious middle class. Pliny ranked it among the stones 'known to all.'

Sardonyx has largely disappeared from mainstream jewelry. Synthetic materials can replicate its banding without the cost or difficulty of carving natural stone. But the few remaining glyptographers — cameo carvers — still prize sardonyx for its workability and contrast. The stone that carried Roman imperial propaganda now carries portrait cameos sold in the workshops of Torre del Greco, near Naples.