Onyx is a cryptocrystalline form of quartz. The colors of its bands range from white to almost every color (save some shades, such as purple or blue). Commonly, specimens of onyx available contain bands of colors of white, tan, and brown. With its fleshtone color, onyx can be said to resemble a fingernail. Some onyx is natural but much is produced by the staining of agate. In correct usage, the name refers to a black and white banded variety of Agate, or sometimes a monochromatic agate with dark and light bands.