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.
Rutile is a mineral composed primarily of titanium dioxide. Rutile has among the highest refractive indices at visible wavelengths of any known crystal, and also exhibits a particularly large birefringence and high dispersion. Small rutile needles present in gems are responsible for an optical phenomenon known as asterism. Asteriated gems are known as "star" gems. Star sapphires, star rubies, and other "star" gems are highly sought after and are generally more valuable than their normal counterparts.