Sapphire glass. Similar to jadeite, the hardness is 9, and various colors can be produced by adding various chemical elements. Colorless is commonly used now. Its advantages are higher hardness and higher price than ordinary glass, and it is mainly used to make the mirror of high-end watches.

Sapphire glass/ruby glass has good thermal characteristics, excellent electrical and dielectric properties and chemical corrosion resistance. It has high temperature resistance, good thermal conductivity, high hardness, infrared transmittance and good chemical stability. Therefore, it is often used to replace other optical materials to make optical elements and infrared-transmitting optical windows, which are widely used in infrared and far-infrared military equipment, such as night-vision infrared and far-infrared sights, night-vision cameras and other instruments and meters in satellite and aerospace technology, as well as windows of high-power lasers, various optical prisms, optical windows, ultraviolet and infrared windows and lenses, observation ports for low-temperature experiments, etc., and has been fully applied in high-precision instruments and meters in navigation and aerospace.