The crystal is a clear cover that fits over the watch face, protecting the interior from dirt, scratches and other damage. Typically this will be made of glass or plastic.
The type of crystal used varies, and is usually determined by the overall price of the watch. For example, the most affordable material used for the crystal is acrylic. This light and inexpensive plastic is really hard wearing and is therefore often used in sports watches since it doesn't shatter or crack. However, it can scratch easily compared to some other alternative materials – so think carefully about your lifestyle and the level of durability you need when purchasing a watch.
Mineral glass is more scratch-resistant than acrylic, but can crack or shatter under extreme conditions. It is made by treating regular glass with either heat or chemicals so that it can withstand rough treatment.
In most luxury watches, the crystal of choice is sapphire crystal. This synthetic compound has the same physical, chemical and optical properties of natural colourless sapphire. It is favoured by high-end watch manufacturers because of it's resistance to scratching – only a diamond or another sapphire can scratch its surface. However, sapphire crystal shatters more easily than acrylic and should not be treated as though indestructible.