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.