Corundum

This is the mineral Corundum. It’s exceptionally hard and is the defining mineral for hardness rank number 9 on Mohs hardness scale. Thus, Corundum can scratch minerals ranked 1-8, but not number 10, which is Diamond. The specific gravity of Corundum is 3.9 to 4.1. Corundum is made from only 2 elements, Aluminum and Oxygen, and its chemical formula is Al2O3. It streaks white, has no cleavage, and belongs to the Oxide chemical family. Oxides are defined by the presence of Oxygen chemically bound to one or more metals. In Corundum's case, that metal is Aluminum. The mineral often exhibits a vitreous luster but may also display an adamantine luster. Adamantine specimens have a high refractive index and thus intensely reflect light, thereby creating a brilliant glittering and sparkling effect. Diamonds are some of the best-known examples of minerals displaying an adamantine luster. Corundum’s hardness makes it especially useful as an abrasive. It’s commonly seen in sandpaper, grinding wheels, and cutting and polishing equipment. It also has a very high melting point, making it useful in refractories where it can withstand intense heating. Perhaps the best-known use of Corundum is as a gemstone. Gemstones are simply minerals that have been cut, ground, and polished by lapidaries. Lapidaries are the people who make the pretty gems that you may be wearing right now on a ring, bracelet, or pendant. When a lapidary cuts, shapes, and grinds the mineral Corundum, it becomes a red ruby or a blue sapphire. The resulting gem is much more valuable than the initial mineral because the lapidary transforms it into a gemstone with an appealing shape and a high adamantine luster.