Aluminum reacts with oxygen to form a layer of aluminum oxide on the outside of the metal, according to HowStuffWorks. This thin layer protects the underlying metal from corrosion caused by oxygen, water or other chemicals. Aluminum burns in oxygen with a white flame to form the trioxide aluminum (III) oxide, says WebElements. Chemists write the chemical reaction between aluminum and oxygen as: 4Al+ 3O2 = 2Al2O3.
In nature, aluminum exists in various compounds, according to HowStuffWorks. Bauxite is a rock-hard mineral used in producing aluminum. The process to separate it is complex. Pure aluminum is lightweight and soft. Metallurgists often combine aluminum with other metals to form alloys. Mixing aluminum with copper or magnesium makes it stronger, and the addition of manganese improves aluminum's resistance to corrosion.
Corundum is the natural form of aluminum oxide. According to About.com, corundum occurs in rocks that are low in silica, particularly in schists altered by aluminum-bearing fluids and altered limestones. About.com also states that corundum is actually the second-hardest natural substance, after diamond. Industries often use a mixture of corundum and magnetite called emery as an abrasive and in sandpaper. Pure corundum is clear, but impurities give it various colors, including blue, red, brown, violet and yellow. In gemstone qualities, red corundum is a ruby, and all other colors are sapphires.