Onyx

Early in the first century AD, a gem-cutter in the circle of Dioscurides, engraver to the emperor Augustus, took a double-layered piece of Arabian onyx and carved down through its white band to the bluish-brown ground beneath. The result, the Gemma Augustea, now in Vienna’s Kunsthistorisches Museum, is among the largest cameos to survive from antiquity: nineteen by twenty-three centimetres of emperors, gods and captives, drawn entirely by the boundary between two layers of a single stone.

Onyx made this art possible because of what it is, chalcedony whose bands run straight and parallel, so the cutter always knows where the white ends and the dark begins. On that certainty Rome built a craft of cameos and signet seals; Pliny the Elder records both the stone’s use for seals and the treatments used to deepen its colour.

A dial asks for the opposite gesture: not two layers played against each other, but one dark stratum sliced thin and polished until the surface itself is the ornament.