What is a Watch Crystal: The crystal is the "window" though which one views the dial of a watch. There are several primary materials from which this pane is made.
Very inexpensive watches will likely have a "crystal" made of some type of plastic. Above that, the material you will likely encounter is a "mineral" crystal.
"Mineral" crystals are synthetic and very durable materials that can be produced affordably and are both scratch and shatter resistant. Its affordability often makes mineral the crystal of choice for lower-end watches, but its durability, especially when produced in heavier, more expensive versions, results in it being used on rugged sport and dive watches, including some fairly expensive examples, in lieu of sapphire.
Sapphire refers to lab-grown synthetic sapphire, which is contoured the way an optical lens is ground. It is prized for being the most scratch-resistant material currently in use for watch crystals, being rated a 9 on the Mohs hardness scale, on which only a diamond, ranked at 10, is higher and harder. Of course the name "sapphire" also offers some upscale cache, making it the material of choice for most high-end watch crystals. The only downside is that while being highly scratch resistant, sapphire crystals can be prone to shattering.