Google search reveals the following: suggest you follow the link and read for yourselfCrystal is glass now, lesson learned. GOT IT
What is a watch crystal?
A watch crystal is a transparent cover that protects the watch face. Note that, coincidently, the word “crystal” is also used to denote the tiny piece of quartz that serves as an oscillator in a quartz watch. These two types of crystals have nothing to do with each other. The latter is usually called a “quartz crystal” to prevent confusion.

Watch crystals
What is a watch crystal? Clear up your questions about crystals with this brief primer.