Aluminium oxide ( corundum , emery , sapphire / ruby )

Next to the main aluminium ore, bauxite, an assemblage of aluminium hydroxides, aluminium oxides and other minerals (gibbsite, diaspore, boehmite, kaolinite, quartz, hematite, goethite, rutile, anatase, and alumogel), aluminium oxides find direct application in diverse industrial processes (e.g., refractory, abrasives, and watch-making industries) as well as in the gemstone sector. Corundum (Al2O3, alternatively: alumina) is the oxygen compound of aluminium with molar mass M = 101.96 g/mol, density ρ = 3.97 g/cm, melting point TM = 2015 °C, and boiling point TB = 2980 °C. Owing to its hardness (Mohs hardness of 9), corundum is, among other uses, processed to (jewel) bearings in measuring instruments or watches and abrasives. Corundum can be produced artificially via elaborate and expensive techniques, i.e., hydrothermal and flux-growth syntheses, Verneuil process. From an economic point of view, corundum synthesised in the Verneuil process proves to be the most interesting, e.g., for sapphire glasses in the watch-making industry. Sintered α-Al2O3 (also: sintered corundum, sintered alumina) is used as refractory material in furnace coatings or laboratory equipment. γ-Al2O3 is used as adsorptive agent, as catalyst or catalyst substrate, and often in special ceramics. New sintering methods permit the application of aluminium oxide in the production of exceedingly strong and break-resistant glasses.