Crystals of quartz have a property known as piezo­electricity. This means that when you apply pressure to a quartz crystal, silicon and oxygen ions shift and cause positive and negative charges to appear on opposite crystal faces. The reverse of this effect also occurs, so that if positive and negative charges are applied, the crystal contracts slightly. Because of this property, a quartz crystal can be made to alternately expand and contract (that is, oscillate) by passing an alternating electrical current (one in which the flow direction of electrons rapidly reverses back and forth) through it. Every crystal has a characteristic frequency, measured in oscillations per second, at which it likes to vibrate. The frequency depends on the size of the crystal, just as the frequency (pitch) of a vibrating bell depends on the size of the bell. When the frequency of the alternating electrical current applied to a crystal comes close to the crystal’s characteristic frequency, the crystal "locks" the electrical current to its frequency. The regularity of this oscillation provides a basis for keeping time. Oscillating quartz crystals thus provide the heart of a quartz watch.

How is a quartz watch made? First, the watchmaker obtains tiny quartz fragments, made by fracturing larger crystals. The crystals are so small that they oscillate at frequencies of between 100,000 and 2.5 million times per second when a current from a small battery passes through them. Devices called "frequency dividers" reduce the vibration to a few beats per second, and these beats either drive gears that then turn the hands of the watch, or control the digital image that appears on the watch face.

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