Quartz crystals have several applications in the electronics industry. However, they are mostly used as resonators in electronic circuits. Quartz is a naturally occurring form of silicon. However, it is now produced synthetically to meet the growing demand. It exhibits the piezoelectric effect. If you apply physical pressure on one side, the resulting vibrations generate an AC voltage across the crystal. Quartz crystal resonators are available in many sizes according to the required applications.

If you apply an alternating voltage to a crystal, it causes mechanical vibrations. The cut and the size of the quartz crystal determine the resonant frequency of these vibrations or oscillations. Thus, it generates a constant signal. Quartz oscillators are cheap and easy to manufacture synthetically. They are available in the range from a few KHz to a few MHz. As they have a higher quality factor or Q factor, crystal oscillators are remarkably stable with respect to time and temperature.

Source: blog.mide.com