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.