Corundum
Corundum is a crystalline form of aluminium oxide (Al2O3) with traces of iron, titanium and chromium. It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red and padparadscha if a pink-orange, while all other colors are called sapphire. Corundum is a very hard mineral that is commonly cut into gem stones. Corundum in it's natural state is clear, but it often collects impurities forming types such as Sapphire and Ruby. Corundum is the second-hardest hardness on the Mohs Scale.

Because of its hardness, corundum is often used industrially to create things such as abrasives and plastic making machine parts. Corundum usually occurs naturally in mica schist or gneiss.

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