Onyx

Exodus 28 describes two separate sets of engraved stones on the High Priest's ceremonial vestments, and onyx belongs to the set most readers overlook. Rather than one of the twelve breastplate stones representing the twelve tribes individually, two onyx stones were mounted on the shoulder pieces of the ephod, the apron-like garment worn beneath the breastplate, each stone engraved with the names of six of the twelve tribes of Israel, so that the priest symbolically carried the whole nation on his shoulders whenever he wore the garment.

This shoulder placement, distinct from the breastplate proper, is the detail that most consistently gets flattened in popular retellings that lump "the priestly stones" into one undifferentiated set of twelve; the text itself is specific that the shoulder onyx and the breastplate's twelve stones served related but different symbolic functions, representation by name grouped in pairs versus representation stone by stone.

Exodus gives the shoulder stones an explicit purpose rather than leaving it to be inferred: they are stones of memorial, engraved with the names of the sons of Israel in the order of their birth, so that Aaron carries those names before the Lord upon his two shoulders. The verb the text uses for what the priest does with them is to bear, the same word used of carrying a physical load, and commentators have generally taken the choice as deliberate.

The breastplate itself carried something the shoulder stones did not, the Urim and Thummim, placed inside it and used in the narrative to obtain a decision. The vestment as a whole therefore combined a memorial function at the shoulders with an oracular one at the chest, which is a further reason the two sets of stones are not interchangeable.

Independent of its biblical role, onyx's banded structure, alternating layers of typically black and white chalcedony, made it a favored material for glyptic art, engraved gems and cameos, across the Greco-Roman world. A skilled engraver could cut through the top layer to reveal a contrasting color beneath, producing a raised image in one color set against a ground of another, a technique that made onyx and related banded stones the material of choice for the finest seal rings and decorative cameos of antiquity.

That same engravability is almost certainly why Exodus specifies onyx for the ephod stones in the first place: a name needed to be cut clearly and durably into the surface, and onyx took an engraver's tool well enough to serve as a functional signet as much as a symbolic gem.

The surviving masterpieces of the technique are cut in sardonyx, onyx banded with reddish sard rather than with white, and they are large: the Gemma Augustea in Vienna and the Great Cameo of France in Paris are both multi-figure imperial compositions carved across the layers of a single stone, and both are the work of an expensive court workshop rather than of a jobbing seal cutter.

Below that level the same skill served an ordinary legal purpose. A signet had to leave an impression in wax or clay that could be recognised and not easily counterfeited, so the qualities that made banded chalcedony good to carve, hardness, fine grain and a predictable layer structure, were practical requirements before they were aesthetic ones.