High-temperature sintering of alumina, said artificial corundum or gems can be prepared by mechanical bearings or Diamond watches. Aluminium oxide is also used as high-temperature refractory, refractory brick, crucibles, pottery, artificial stones, alumina and aluminium materials. Calcination of aluminium hydroxide can be prepared by γ-ray-Al ₂ o ₃. Gamma-Al ₂ o ₃ has strong adsorption and catalytic activity of adsorbents and catalysts. Main component of corundum α-Al ₂ o ₃. Barrel-shaped or cone-shaped trigonal crystal. A shiny glass or adamantine luster. Density of 3.9~4.1g/cm3, hardness 9, 2000 ± 15 degrees melting point. Not soluble in water, insoluble in acids and alkalis. High temperature resistance. Colorless, transparent white, trace amounts of trivalent chromium-containing Red Ruby; containing ferrous and ferric iron or four titanium blue Sapphire; including small amounts of iron oxide dark grey, dark, said Emery. Bearings that can be used as precision instruments, watches diamond, grinding wheels, polishing agents, refractory materials and Electrical insulators. Brightly decorated with precious stones. Laser materials made from synthetic Ruby crystal. Apart from natural minerals, and oxy-hydrogen flame melting aluminum hydroxide preparation.