Quartz crystals are piezoelectric - applying voltage to them will cause vibration. Each quartz crystal resonates at a certain frequency with great precision, and crystals can be cut to vibrate at any needed frequency. This makes them ideal for keeping time in clocks and watches, since a simple battery (as a source of electricity) can replace an intricate sytem of mechanical gears, thus making accurate timepieces affordable. They are also used for frequency control and timekeeping purposes in many electrical and electronic circuits.