Star ruby is a rare variety of ruby that exhibits asterism; a six-rayed star that shimmers over the surface of the stone when it is moved. The star effect is owed to aligned needle-like rutile inclusions. It is the rutile that is responsible for its silky shine. In rough form, ruby is dull and greasy, but when cut and polished its luster is comparable to fine diamond. Ruby is classified as the red form of gem-quality corundum; all other color varieties of corundum are referred to as sapphire. Corundum is the second hardest mineral on the Mohs scale, with a hardness rating of nine. Owing to its superior hardness combined with its rich color and silky shine, fine quality ruby is classed as one of the groups of four so-called "precious" stones, along with diamond, emerald, and sapphire. Ruby gets its name from the Latin word rubber, which means red.