A quartz crystal resonator depends on the piezo-electric effect to work. This effect converts a mechanical stress in a crystal to a voltage and vice versa. In this way the piezo-electric effect converts the electrical impulses to mechanical stress which is subject to the very high Q mechanical resonances of the crystal, and this is in turn linked back into the electrical circuit.

The quartz crystal can vibrate in several different ways, and this means that it has several resonances, all on different frequencies. Fortunately the way in which the quartz crystal blank is cut from the original crystal itself can very significantly reduce this. In fact the angle of the faces relative to the original crystal axes determines many of its properties from the way it vibrates to its activity, Q, and its temperature co-efficient. There are three main ways in which a crystal can vibrate: longitudinal mode, low frequency face shear mode, and high frequency shear. A cut known as the AT cut used for most crystals used in traditional radio and electronics circuits uses the high frequency shear mode.

Vibrational modes of a quartz crystal resonator
(For the sake of clarity, the movements have been greatly exaggerated)