Corundum is the form of mineral from which sapphires and rubies are derived. The mineral is naturally clear, but can have different colors when impurities are present. Transparent corundums are used as gems, and if they’re red, then they’re rubies, while all other colors are called sapphire. The word corundum comes from the Tamil kurundam Due to corundum's hardness (typically 9.0) it is commonly used as an abrasive in machining, from huge machines to sandpaper. Emery is an impure and less abrasive variety, with a Mohs hardness of 8.0. You’ve probably used corundum hundreds of times when you’ve filed your nails with an emery board!
Corundum is actually the second hardest substance on the earth, after the diamond, and as such has many industrial uses. Corundum is crushed, ground and screened to varying grain sizes. The grains are utilized in the manufacture of grinding wheels for flour and rice mills, and workshops. Grinding wheels are manufactured either by a mixture of corundum, clay and feldspar or by using sodium silicate as a bonding agent. Finer grains are utilized for the preparation of grinding-pastes for the automobile industry. Corundum paper and cloth finished to belts, discs, rolls, sheets and other shapes are prepared by giving a suitable coating generally of sodium silicate.
Due to its hardness corundum also finds use in mortars, wire drawing dies, thread guides and gauge blocks. Gem varieties are sometimes used for pivot supporters in delicate scientific instruments, as jewel bearing in watches.
Corundum in small quantities is used in the manufacture of special sparking plugs, mufflers, pyrometer tubes, rods and insulators for vacuum tubes of all kinds. Finely ground corundum, passing through 200 mesh, is used for the manufacture of the above products by giving them a bond of clay.