Onyx << ON ihks >> is a term used loosely to apply to a banded carbonate rock and also to agate, a fine-grained variety of quartz (see Agate).
Ordinary onyx of quartz occurs in such color combinations as black and white, green and white, and red and white. Sardonyx is a reddish-brown and white onyx. Onyx is hard and takes a high polish. It is widely used in the carving of cameos and intaglios. Today, jewelers refer to single-color agate as onyx. When they speak simply of onyx, they mean the black stone.
Onyx marble (Mexican onyx) is a variety of calcite rock found on the walls of caves. Mexican onyx shows a banding like that of agate, but it is much coarser. The colors of Mexican onyx range from white to green, red, and brown. Much of this soft onyx marble is cut into gemstones, colored with an aniline dye, then set in inexpensive native silver jewelry. The stones are brittle and not durable. Mexican onyx is also used as a decorative stone.
