Sapphire (cryst. Al2O3)
is free of absorption in the range 240nm-4µm. Optical components generally have C-cut (001-orientation).
It stands out by hardness, strength, chemical and thermal stability and is often used as high pressure and high temperature window material and for view ports. Compared to Fused Silica it has 20 times higher heat conductivity. In addition it is insoluble in most common acids.