Gemstones are minerals that are cut and polished to make beautiful "stones" for jewelry and other adornments. Humans traditionally consider diamonds, rubies, sapphires, and emeralds to be the four "precious" gemstones, with all other gemstones called "semiprecious." Many gems are transparent, meaning they allow some or most of the incident light to pass through them. To make a stone sparkle, gemcutters cut and polish the facets, or flat outer surfaces, of a gemstone to boost the number of times a light beam will reflect internally before leaving the stone and reaching the eye. Some semiprecious stones, such as malachite and lapis lazuli (a rock containing the mineral lazurite), are opaque to light; gemcutters polish them to a high gloss, without facets. This shiny, rounded type of gemstone is called a cabochon. Corundum (ruby or sapphire) specimens that contain tiny fibers of other minerals also may be cut into cabochons to create a luminous "star" effect on the stone's surface.