(n.) Chalcedony in parallel layers of different shades of color. It is used for making cameos, the figure being cut in one layer with the next as a ground.
Example Sentences:
(1) I have no idea how much she spends in Tesco, or whether there is any onyx in her home fitments.
(2) In response to the need for automated assistance with this class of problems, we have devised a computer program called ONYX that combines decision-theoretic and artificial intelligence approaches to planning.
(3) Cregan should have been easy to spot: he was missing an eye and would wear a mismatched onyx eye in a startling black in its place.
Sardonyx
Definition:
(n.) A variety of onyx consisting of sard and white chalcedony in alternate layers.