Aluminum Oxide is a very hard naturally occuring compound consisting of 2 aluminums bonded to 3 oxygens. In other words: Al2O3.

Aluminum oxide accurs naturally in the Earth's crust as bauxite, corundum, or as a ruby, saphhire, and several other gemstones. Bauxite, which is mined and refined to produce a purified, calcined alumina in the form of a fine white powder. This powder is smelted to manufacture aluminum products. Corundum which is an extremely hard form of aluminum oxide, so hard that it is the second hardest material on the Mohr's hardness scale. And according to this scale it is only a 1/4 as hard is diamond(which is extremely durable). However synthetically made aluminum oxide, also known as Alumina, and fused aluminum oxide, Alundum, is not as extremely hard as corundum is. If you've ever tried to cut a lot of metal or wood with an aluminum oxide cutting wheel, you'll soon find out how weak it is, as the stress of grinding the material makes it brittle like a potato chip.

Alumina is commonly used as an abbrasive for grinding metal, sanding, and even for certain cutter wheels. Heated alumina has a porous structure that easily absorbs moisture and vapors and is therefore used to dehydrate liquids and gases. Aluminum sulfate, or activated alumina, or clay, with sulfuric acid, is important in paper manufacture as a color binder and a filler. Other alumina compounds produce alums and are used for waterproofing fabrics and as the antiperspirant in commercial deodorants.