Corundum is probably one of the most durable commercial gemstones, taking into account its hardness of 9 on the Mohs scale. It is popular for its gem variations, viz. sapphire and ruby. The rock-forming mineral is a crystalline form of aluminum oxide that usually contains traces of chromium, vanadium, titanium, and iron. It is found in sedimentary, metamorphic, and igneous rocks and has a hexagonal crystal structure. If opaque, pure corundum is pale white, and if transparent, it is clear and colorless. Although it is commonly found as opaque crystals, corundum comes in many different colors, i.e. in almost all rainbow colors.