At the core of almost all modern digital systems is a high accuracy timebase based on a crystal oscillator. Crystal oscillators use the property of piezoelectricity to create waveforms with excellent frequency accuracy based on the mechanical vibration of a precision machined crystal.

Certain crystals, most notably quartz, exhibit the property of piezoelectricity. If force is applied to the faces of a piece of crystal it causes a voltage to be
generated from the crystal. Conversely, if an electrical voltage is applied to a crystal it causes the crystal to deform mechanically.

http://www.cardinalxtal.com/docs/notes/cardinal_oscillator_basics.pdf

In the series resonant circuit the crystal is placed in a series path around the amplifier. The frequency of oscillation can be altered slightly by changing the capacitive loading on the crystal element. In this circuit the capacitance
is varied using a varactor diode whose capacitance varies with an applied control voltage.

The frequency stability of the oscillator is a function of many factors. First, the “cut” or orientation of the crystal blank relative to the major axes of the entire
crystal. Other factors include temperature, power supply quality, mechanical vibration, gravity, aging, and loading. Designers must be able to measure and characterize these effects.

Also Read: Multi Wave Oscillator, Crystal Oscillator, Voltage Controlled Oscillator, Colpitts Oscillator, Oscillator Circuit, Mcclellan Oscillator, Clock Oscillator, Hartley Oscillator.