Not to be confused with quartz crystal, which is used as a time regulator on most battery-operated watches, the crystal refers to the clear covering on the face of the watch. Watch manufactures use three different types of materials. The style, type and, of course, cost determine what type of material is used for a particular model.

Also called acrylic glass, plastic offers the benefit of being inexpensive, light and durable. These qualities make it ideal for sports and children’s watches. While plastic does not shatter or crack, it does scratch very easily, spoiling the appearance of a nice-looking timepiece. A professional jeweler can buff out light marks, but deep scratches can allow moisture to seep into the mechanism and condense on the inside. Replacement is often the recommended remedy in these cases.

Mineral crystal refers to ordinary glass that has been heat or chemically treated to withstand bumps and scratches. While it is more resistant to scratches, it is less flexible than plastic and can crack or shatter. While rare, this can happen under extreme hot or cold conditions when the glass is hit bluntly such as after entering a warm room on a very cold day.

The term can be misleading. The crystal is not shaped from an actual sapphire but is a synthetic compound with the same properties of its natural counterpart. Although it’s not unbreakable, it is the choice of ultra-luxury watch manufacturers due to its ability to withstand scratching, cracks and breakage more than glass or plastic. Only a diamond or another sapphire can scratch the surface. Due to its durability, special diamond-tipped cutters are needed to cut and shape the glass, making it the most expensive watch crystal available. Rolex uses sapphire crystals in many of its models, while Seiko and Citizen use it on some of their high-end products.

The type of crystal can influence the price and value of a watch. While it is almost impossible to determine the type of watch crystal by looking at it, a simple scratch test can be performed. While this method is not recommended or foolproof, it is a quick and easy way to test the crystal. Simply take a razor blade and try to put a small scratch on an inconspicuous area of the glass. If the area remains smooth and unmarked, the crystal is, most likely, sapphire.

If still in doubt or fearful of damaging the watch, visit your neighborhood jeweler. This highly trained professional can examine your watch and advise you on its construction, value and care.