Onyx is a variety of chalcedony in bands or strata of white, gray, and black, translucent to opaque, and generally found where agates abound.

The layers or bands are in even planes, and the colors, white and black, white and brown, or brown, white, and black, alternate. This stone is largely used for cameos, the base being usually of black or brown, and the engraved or upper part white- or cream-colored.

When one or more layers are of carnelian or sard, the stone is called sard-onyx. Sard is a rich brown color inclining to red, and when held against the light shows a red hue.

Onyx and sard-onyx are often artificially improved by boiling the stones in honey, oil, or sugar water, and then in sulphuric acid. The acid carbonizes the sugar or oil which the stone has absorbed and gives it a deeper color.

For red, protosulphate of iron is added, and for a blue color to imitate lapis lazuli, yellow prussiate of potash is added to the protosulphate of iron.

Only the porous parts of the stones, usually the dark parts, absorb the sugar or oil, and so aid the contrast of the colored with the white layers.