Synthetic stones are created artificially in a factory or laboratory, but have the same physical, chemical and optical characteristics as their natural counterparts. These stones usually have high luster, and is difficult to tell the difference from natural stones with the naked eye.
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.