The cover over the watch dial is called the crystal. There are three types of crystals commonly used in watches: acrylic crystal is an inexpensive plastic that allows shallow scratches to be buffed out. Mineral crystal is composed of several elements that are heat-treated to create an unusual hardness that aids in resisting scratches. Sapphire crystal is the most expensive and durable, approximately three times harder than mineral crystals and 20 times harder than acrylic crystals. A non-reflective coating on some sport styles prevents glare.