Sapphire glass crystal extreme surface hardness, high thermal conductivity, high dielectric constant and resistance to common chemical acids and alkalis which making them ideal for demanding applications (such as laser systems). Sapphire is the second hardest crystal next to diamonds and, because of their structural strength, sapphire windows can be made much thinner than other common dielectric windows with improved transmittance. Chemically, sapphire is single crystal aluminum oxide (Al2O3) and is useful in a transmission range from 0.2 - 5.5μm.