This is synthetically produced Sapphire. Single crystal Sapphire has a high thermal conductivity, volume resistivity, hardness and wear resistance, as well as a high upper use temperature. The energy band gap is large (10eV). Sapphire is a transparent material having useful transmission of wavelengths in the range 0.2 to 5.5 microns. It is much stronger than glass and can be used as viewing windows in high pressure and vacuum applications.