ὄνυξ

Onyx comes directly from Greek ὄνυξ (ónyx), meaning 'claw, nail, hoof.' The word named the gemstone because of a visual resemblance: the white veins running through dark chalcedony onyx recalled the white lunula (crescent) at the base of a human fingernail, or the white tips of fingernails and claws against darker flesh. The naming represents the same analogical logic that gave garnet its pomegranate name: the gem classified by its closest visual equivalent from the body or the natural world. Greek ónyx meant fingernail first and gemstone second, though both usages appear in classical texts and the priority of one over the other is difficult to establish. The Latin onyx carried both meanings into medieval usage.

Ancient myth explained the name poetically. According to one tradition preserved in several ancient sources, the stone originated when Cupid trimmed Aphrodite's fingernails while she slept on the banks of the Indus River. The clippings fell into the river, and since no part of a divine body could perish, the Fates transformed them into stone. What had been the goddess's nails became the gem that bore the nail's name. The myth is clearly etiological — invented to explain an already-existing linguistic connection — but its detail (the Indus River) accurately reflects that fine onyx did come from the East, including from India, and that ancient trade routes brought it west through Persian and Greek middlemen.

The ancient world valued onyx primarily for hardness and for its suitability to carving. Onyx and sardonyx (a reddish-brown and white variety) were the preferred stones for cameos and intaglios — the carved gems used as seals, rings, and decorative objects. The alternating layers of different-colored chalcedony allowed skilled engravers to carve images that used the natural color contrast: a white face carved against a dark background, a dark figure against a white ground. The Roman cameo tradition produced some of the finest small-scale carving in the ancient world, and the material that made it possible was onyx. The nail-named stone sustained an entire art form.

Modern onyx is complicated by the word's loose commercial application. True geological onyx is a variety of chalcedony (microcrystalline quartz) with parallel banding in black and white or brown and white. 'Black onyx' in commercial jewelry is often dyed chalcedony or dyed agate — chalcedony with irregular rather than parallel banding. 'Mexican onyx' and 'Pakistani onyx' are frequently calcite, a completely different mineral. The word 'onyx' has been stretched by the gem trade to cover visually similar stones regardless of their actual mineralogy. The ancient nail-word, already imprecise in its original application, has become even looser in commercial usage, covering a family of dark-banded stones by appearance rather than by chemistry.