The crystal is the transparent protective covering fitted tightly over the face of the watch. Since it is exposed to all sorts of shocks and blows, the crystal should be as hard as possible. Softer materials will scratch and break easily, which will make the watch less attractive and possibly unreadable, and if shattered, they can damage the watch beyond repair. The crystal is held in place by the bezel.
A crystal can be made from any clear material, from plastic to synthetic sapphire, which is the hardest and most expensive crystal available. The following chart lists popular crystal types and their relative hardness.