The most abundant metallic element in the earth's crust, with a content of 8.3%. Mainly found in aluminosilicate ores, as well as bauxite and cryolite. Aluminum oxide is a white amorphous powder that comes in many variations, the most familiar of which are α-Al₂O₃ and β-Al₂O₃. Corundum that exists in nature belongs to α-Al₂O₃. Its hardness is second only to diamond. It has a high melting point and is acid and alkali resistant. It is often used to make some bearings, abrasives, and refractory materials.
For example, corundum crucible can withstand high temperatures of 1800°C. Al₂O₃ comes in a variety of colors due to the different impurities it contains. For example, those containing trace amounts of Cr(III) are red and are called rubies; those containing Fe(II), Fe(III) or Ti(IV) are called sapphires.
Aluminum is a light metal with chemical symbol Al and atomic number: 13. The content of aluminum in the earth's crust ranks third after oxygen and silicon. It is the most abundant metallic element in the earth's crust, and its reserves rank second among metals. Among metal types, it is the second largest category of metal after steel.