A crystal oscillator is an electronic device that uses the mechanical resonance of a physical crystal of piezoelectric material to create an electrical signal with a very precise frequency. It is an especially accurate form of an electronic oscillator. The crystal is usually made of quartz, but can also be made of rubidium or ceramic. This frequency is used to keep track of time (as in quartz wristwatches), to provide a stable clock for digital circuits, and to stabilize frequencies for radio transmitters. Crystal oscillators are the most common source of time and frequency signals.