Onyx, a variety of quartz, analogous to agate and other cryptocrystalline varieties, such as carnelian, jasper, chrysoprase, and bloodstone. It is composed of layers of different-colored carnelian, much like banded agate in structure, but the layers are in even or parallel planes, and the banding therefore straight; on which account it is well adapted to the cutting of cameos, and was much used for that purpose by the ancients. The colors of the best are perfectly well defined, and are either white and black, or white, brown, and black; the finest specimens are brought from India. Many of the cryptocrystalline varieties of quartz, particularly carnelian, contain variable small portions of opal, a hydrous quartz (see Opal), which enhances their lustre and value. Sardonyx has a structure like onyx, but is composed usually of alternate layers of white chalcedony and carnelian (sard), although the carnelian may be associated with layers of white, brown, and black chaleedonv.