Sapphire glass is used in Skeleton (AVE-64) and Tourbillon (STL-117) is known to be the definitive material for transparency and durability. The second strong material on the earth.
Coated with anti reflective coating.
Mohs Scale ≈ 9
Vickers hardness ≈ 2000 HV
Multilayer of Anti-reflection coating is applied to ensure you can read the dial clearly.