Agate is a cryptocrystalline variety of silica, chiefly chalcedony, characterized by its fineness of grain and brightness of color. Although agates may be found in various kinds of rock, they are classically associated with volcanic rocks and can be common in certain metamorphic rocks.
Onyx is a banded variety of the oxide mineral chalcedony. The colors of its bands range from white to almost every color (with the exception of some shades, such as purple or blue). Commonly, specimens of onyx contain bands of black and/or white.