The glass is for protecting the dial, it has several different materials:

Plastic acryl glass is so called acrylic. It is soft and easy to shape. The weakness is easy to be scratched and the diaphaneity is not so good. The advantage is it’s really flexible and hard to break, that’s why it’s also called safety glass. The plastic glass is easy to be scratched, but you can always use toothpaste to wear it off.

Mineral crystal glass is so called artificial crystal. The diaphaneity is good, and the level of limpidity and rigidity is according to the polish technique.

The rigidity is almost as hard as diamond. The diaphaneity is good, the friction is small, and the glass is heat-resistance and corrosion resistance but easy to crack.