R.B. Aye, here you be ye ruddy dustmop! :)
http://www.surmet.com/technology/alon-optical-ceramics/
https://en.wikipedia.org/wiki/Aluminium_oxynitride
Chemical formula (AlN)x·(Al2O3)1−x, 0.30 ≤ x ≤ 0.37
Aluminium oxynitride is used for infrared-optical windows, with greater than 80% transparency at wavelengths below about 4 micrometers, dropping to near zero at about 6 micrometers.[8] It has also been demonstrated as an interface passivation layer in some semiconductor-related applications.[9]
Aluminium oxynitride has less than half the weight and thickness of glass-based transparent armor.[10] Aluminium oxynitride armor of 1.6-inch (41 mm; 4.1 cm) thickness is capable of stopping .50 BMG armor-piercing rounds, which can penetrate 3.7 inches (94 mm; 9.4 cm) of traditional glass laminate.[8][11]
In 2005, the United States Air Force began testing aluminium oxynitride-based armor.[12]