A quartz crystal inside the oscillator is the resonator. It could be made of either natural or synthetic quartz, but all modern devices use synthetic quartz. The crystal strains (expands or contracts) when an electrical voltage is applied. When the voltage is reversed, the strain is reversed. This is known as the piezoelectric effect. Oscillation is sustained by taking a voltage signal from the resonator, amplifying it, and feeding it back to the resonator. The rate of expansion and contraction is the resonance frequency, and is determined by the cut and size of the crystal. The output frequency of a quartz oscillator is either the fundamental resonance or a multiple of the resonance, called an overtone frequency. A typical Q for a quartz oscillator ranges from 10^4 to 10^6. The maximum Q for a high stability quartz oscillator can be estimated as Q = 1.6 x 10^7/f, where f is the resonance frequency in MHz.
Environmental changes of temperature, humidity, pressure, and vibration can change the resonance frequency of a quartz crystal, but there are several designs that reduce these environmental effects. These include the TCXO, MCXO, and OCXO. These designs (particulary the OCXO) often produce devices with excellent short-term stability. The limitations in short-term stability are due mainly to noise from electronic components in the oscillator circuits. Long term stability is limited by aging. Due to aging and environmental factors such as temperature and vibration, it is hard to keep even the best quartz oscillators within 1 x 10^-10 of their nominal frequency without constant adjustment. For this reason, atomic oscillators are used for applications that require better long-term stability and accuracy.