The crystal is a clear cap that protects the face (the dial) of the watch from moisture, debris, and dust.
Sapphire
A sapphire watch crystal is one of the most durable, scratch resistant crystal that a watch can have. Its exceptional hardness, 9 out of 10 on the Mohs’ hardness scale, made synthetic sapphire perfect for use in armored vehicle windows, as military helicopter glass, as well as in military body armor. A synthetic sapphire is created by sintering and fusing agglomerated aluminium oxide in an inert atmosphere.
Acrylic
Acrylic crystal is a plastic material used as a watch crystal. It is durable, easy to maintain, fairly cheap to replace, and most minor scratches can be buffed out with ease.
Mineral Glass
Mineral glass is a scratch and impact resistant form of glass. It is less expensive than a sapphire crystal and ranks a 7 on the Mohs’ scale of hardness.