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Ruby Corundum Crystal

Corundum is a crystalline form of aluminum oxide with traces of iron, titanium, and chromium. Red corundum crystals are called Ruby, while all other colors are called sapphire. These crystals are not considered gem quality, although they can be cut into cabochons and some may display a "star" pattern when polished.

Due to corundum's hardness, 9.0 on the Mohs scale, it can scratch almost every other mineral. It is commonly used as an abrasive, on everything from sandpaper to grinding wheels. Corundum occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic rocks. It can also occur in igneous rocks. Hexagonal crystal system.