Glass is one of the most important parts of a watch. There are 4 major types of glass. The quality and the price will depend on its solidity.

1. Sapphire glass is the most solid and expensive. It is extremely reliable. Thecase and the wristlet may peel off, but sapphire glass will look as if it is new.
2. Crystal is the second best material. It is generally used in watches of middle-class.
3. Mineral glass is a bit inferior to the previous glasses in solidity. Actually both mineral glass and crystal can be scratched by some steel things.
4. Plastic is generally used in cheap watches. It can be scratches easily by anything. It is never used in watches with a high waterproof level.

There some other types of glass as well. For example Seiko uses Sapphlex, which is a double-layer glass with sapphire as top layer. It is less expensive than sapphire but it is practically as good in quality as the latter.