Black onyx is a solid-black variety of chalcedony, a microcrystalline form of quartz. Most commercial black onyx is naturally pale agate that has been dyed and heat-treated to an even, glossy black — a long-accepted practice. In Southwestern jewelry it provides deep black contrast in inlay and against silver.
Onyx is a banded variety of chalcedony — quartz built from microscopic crystals. Strictly, “onyx” refers to the parallel-banded form, but in jewelry the word almost always means solid black onyx, a dense, even black stone that takes a mirror polish and feels cool and substantial, like the quartz it is.
Here is the part worth knowing: most black onyx has been treated. Naturally solid-black chalcedony is uncommon, so for well over a century pale gray agate has been dyed and gently heated to produce a uniform, permanent black. This is a stable, traditional, fully accepted treatment — the stone is still genuine chalcedony — but it is a treatment, and an honest seller will say so.
Black onyx is easy to confuse with jet, another black material used in Southwestern jewelry. Jet is organic — a form of fossilized wood, like a hard coal — and is lighter and warmer to the touch, while onyx is mineral quartz, heavier and cooler. Both give the deep black that balances turquoise and shell in inlay, but they are different materials.
In Zuni and Navajo inlay, black onyx supplies the strong dark element of a design, framing or contrasting with turquoise blue, coral red, and the white of shell. Its even color and durability make it ideal for clean channel and mosaic work, and it is also cut into beads and cabochons.
Because the black is usually a dye treatment, accuracy is what matters: genuine treated black onyx is a fine material at its price, distinct from a black glass or plastic imitation. A reputable seller identifies it correctly and attributes the piece to a named maker.