The covering of the watch dial. Acrylic, mineral, and sapphire are the 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 helps resist scratches. Sapphire is the most expensive and durable crystal, approximately three times harder than mineral crystals and 20 times harder than acrylic crystals.