Sapphire crystal (Al2O3) - the hardest oxide crystal. As an excellent filtering material, sapphire has good transmittance in the ultraviolet, infrared, and visible light bands. Sapphire has good transmittance in a wide spectral range from ultraviolet (about 190 nm) to mid infrared (about 5000 nm). The Mohs hardness of sapphire is about 9, second only to diamond, which gives sapphire window plates excellent wear and scratch resistance. It has a series of excellent physical and chemical properties, such as high strength, high hardness, high temperature resistance, wear resistance, corrosion resistance, good light transmission performance, and excellent electrical insulation performance. Therefore, sapphire crystals can be widely used in fields such as infrared and ultraviolet windows for aerospace instruments, temperature measurement windows for blast furnaces, protective covers for solar cells, watch mirrors, infrared military devices, scientific research, civilian industry, and window materials for high-intensity lasers.