Natural stones (stones that can be used in jewelry after cutting and polishing) are well-known for their vibrant colors, but are not one of the four precious stones (diamond, emerald, ruby, sapphire).
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.