Onyx and agate: banded stones in the Bible, cameos and lapidaries

Onyx and agate belong to the same mineral family, chalcedony, a microcrystalline variety of quartz. They differ above all in appearance: agate shows coloured bands that are often curved and concentric, onyx parallel, sharply defined layers, usually white and black. These very layers made them precious to ancient engravers, who exploited their superimposed colours, and made them the object of very different, sometimes opposite, beliefs.

Onyx in the Bible and in cameos

In Exodus the high priest wears on his shoulders two stones engraved with the names of the twelve tribes of Israel; Jerome's Vulgate calls them lapides onychinos. The identification of the Hebrew stone, the shoham, remains debated, but the Latin translation helped give onyx a sacred dignity throughout the Middle Ages, when the Vulgate was the standard biblical text in the West.

In the Hellenistic and Roman world onyx and sardonyx, with white and brown layers, were the materials of the great cameos. The Gemma Augustea in the Kunsthistorisches Museum in Vienna and the Farnese Cup in the National Archaeological Museum of Naples are among the masterpieces of this art, in which the pale relief emerges from the dark ground with almost painterly effects. The Gemma Augustea celebrates Augustus among the gods, while the Farnese Cup, from the Ptolemaic period, shows an allegorical scene linked to the Nile and bears a Medusa head on its outer side.

The bad reputation of onyx in the lapidaries

Curiously, medieval lapidaries attribute disturbing qualities to onyx. Marbod of Rennes writes that whoever wears it at the neck is visited by sad dreams and night terrors, and that the stone stirs up quarrels and discord. Albertus Magnus, in De mineralibus, repeats these beliefs as received tradition. The name, from the Greek onyx, nail, inspired in modern times a tale in which Eros cut the nails of the sleeping Venus; but no reliable ancient source for this story is known.

The agate of Sicily

Agate takes its name from the river Achates in Sicily, usually identified with today's Dirillo, where, according to Theophrastus and Pliny, it was first found. Pliny notes that it was once highly prized but no longer so in his day, and recalls the famous ring of Pyrrhus, king of Epirus, in whose agate the natural veins depicted Apollo with his lyre and the nine Muses, each with her own attribute.

To the medieval lapidaries, on the other hand, agate seemed beneficial. Marbod credits it with the power to make its wearer pleasing and persuasive and to defend him from adversity, and in many later traditions it became an amulet against the bite of venomous animals. The natural patterns of its bands, which looked like landscapes or figures, fed the idea that the stone held a hidden order.

The two stones show how far symbolic meanings can diverge even between similar materials. Onyx, dark and regular, was linked to melancholy and dreams; agate, rich in natural patterns, to grace and protection. Both, however, owe their most lasting fame to the hand of engravers, who turned their layers into images, seals and cameos still admired today in European museums.