Watch glasses have to be robust against scratching and rupture.

Scratch resistance is measured on the Mohs hardness scale in steps of 1-10. The following types of glass are used with watches

• Plexi / acrylic glass
  (polymethyl methacrylate (PMMA)); 3-4 Mohs, little scratch resistance, but unbreakable. Scratches can be polished with special acrylic polishing paste. From this material extremely arched (domed) watch glasses can be produced.

• Mineral glass
  5 Mohs. Mineral with hardened surface 6-7 Mohs (harder than stainless steel).

• Sapphire crystal
  is an artificially produced crystal 9 Mohs (close to diamond, extremely scratch resistant and break-resistant).