The crystal of a watch is the “glass” that you look through to see the dial on the front of the watch (or to see the movement through the back of the watch). There are two materials that I use for the crystal: these are mineral crystal and sapphire crystal.
The standard is called mineral crystal and is essentially a slightly hardened form of regular glass. It can be scratched and chipped if it comes into contact with something harder than itself. Usually this means anything hard like metal or stone (brick, concrete, etc.) A mineral crystal comes standard on most of my watches, except for watches made from precious metals like silver or gold. In those watches I use sapphire crystals.
Sapphire crystal is the other choice I offer in most of my watches. A sapphire watch crystal is made from synthetically grown sapphire. Basically this is essentially the same as sapphires that come out of the ground, just colourless rather than blue, and artificially made rather than naturally formed. Sapphire is an extremely hard substance and can only be scratched by something harder than itself. There are not many substances harder than sapphire, the most notable of these being diamond. So, if you’d prefer to have a very scratch resistant surface through which to enjoy seeing the dial and the inner workings of your watch, choose the upgrade to sapphire crystals.