The crystal is the part of the watch that covers the dial and protects it from being damaged. Redwood uses two types of crystals that are commonly used in watches: hardened mineral glass and sapphire.
Hardened Mineral Glass
Hardened mineral glass is made through a process of heating and cooling glass to temper it. This makes it considerably harder than regular glass. Tempering confers scratch-resistant properties to the watch crystal and makes it less likely to splinter.
Sapphire crystal is made from a single piece of synthetic sapphire. It is then cut with diamond blades and carefully polished to create the watch crystal. Sapphire crystal is much harder than mineral glass, making it more resistant to scratches. It ranks at 9 on the Mohs hardness scale (diamonds rank 10) making it one of the hardest materials in existence.
The Redwood Brothers