Corundum is a crystalline form of aluminum oxide (Al2O3) with traces of iron, titanium and chromium. It can be found as a component in rocks as well as in pure crystal form. It is one of the naturally transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems — Ruby and Sapphire. which are scientifically the same mineral but just different colors. Ruby is red and Sapphire is the variety that encompasses all other colors, although the most popular and valued color of Sapphire is blue.
Because of corundum’s hardness (pure corundum is has a hardness of 9.0 Mohs), it can scratch almost every other mineral. It is commonly used as an abrasive on everything from sandpaper to large machines. Corundum is the third hardest natural mineral known to science. The hardest mineral, diamond is four times harder than corundum. The second hardest is Moissanite (Silicon Carbide) at 9.25. The hardness of corundum can be partially attributed to the strong and short oxygen-aluminum bonds. These bonds pull the oxygen and aluminum atoms close together, making the crystal not only hard but also quite dense for a mineral made up of two relatively light elements.
Corundum can be found in stream and beach sands because of its hardness and resistance to weathering. The largest documented single natural crystal of corundum ever found measures about 65×40×40 cm (26×16×16 in), and weighs 152 kg (335 lb).
Corundum can not only be artificially synthesized, but even natural gem stones are often heat treated to enhance their color.
The specimen featured in the picture above was acquired from Pierre Trudel, who recently visited the Gitche Gumee Museum from Quebec, Canada. The smaller specimen shown in close up detail below was donated by Pierre.
Below are a couple of close up photos that I took of this specimen of black corundum.
According to the web page http://www.mineralminers.com/html/corminfo.htm#metaphysical-corundum-information , “Corundum is said to enhance one’s creativity, self-confidence, awareness, intuition and ambition. It is also said to help release negative emotions in constructive ways while giving give inner peace.”