Corundum itself is not a very rare mineral, but transparent gem-quality corundum is extremely rare. Most corundum is opaque to translucent and heavily included, suitable only for industrial use, including the production of abrasives used for sandpaper and machining of metal, plastics and wood, and as of recently, has become very important in the making of mobile phone components. The name 'corundum' comes from the Sanskrit word 'kuruvindam', meaning "ruby sapphire", which are the two distinct members of the corundum family.
Ruby is red corundum, while all other colors are referred to as sapphire. Blue sapphire and red ruby make up 2 of the 'four precious gems', which also includes green emerald and diamond. While there is no definitive distinction between red ruby and sapphire; in many cases, a near-red sapphire may be classed as an inferior color grade ruby. Thus, it is usually more common of practice to trade near-red sapphires as 'high-grade' fancy sapphire, rather than as 'low-grade' ruby. The corundum family includes variations of ruby and sapphire, including color change sapphire, star sapphire and star ruby. Ruby is also found in mixed gems such as ruby-zoisite and ruby-in-fuchsite.