A watch crystal is a transparent cover that protects the watch face. Coincidently, the word "crystal" is also used to describe the tiny piece of quartz that acts as an oscillator in a quartz watch. These two crystals have nothing to do with each other. The latter is usually called a "quartz crystal" to prevent confusion.
Watch crystals are made with three different materials: 1. Plexiglass (a clear, lightweight plastic). 2. Common glass - like that used for windows, and usually referred to as "mineral crystal or Harlex crystal (Seiko)". 3. Synthetic sapphire. Some crystals are made of both mineral and sapphire glass. Seiko, for example, makes some watches with crystals made of mineral glass covered with a layer of synthetic sapphire. Seiko calls this composite material "Sapphlex".
Advantages and disadvantages of materials
Plexiglass is the least expensive. It is also the least prone to shatter and the most prone to scratches. Mineral crystal, even though it has been hardened by a tempering process, is more likely to break than plexiglass. It is also more scratch-resistant. Synthetic sapphire is the most expensive crystal material and the most scratch resistant. Saphhire is also brittle, and shatters more easily than mineral glass or plexiglass.