Sapphires are composed of gem-quality corundum (Al₂O₃). Color appears in sapphires when small amounts of iron and titanium are present. Star stapphires contain needles of the mineral rutile that will display as a six-ray star figure when cut in the unfaceted cabochon (dome or convex) form. The most highly prized star sapphires are blue. Black or white star sapphires are less valuble. Since a ruby is simply the red variety of corundum, star rubies also exist.