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.