@alex >A crystal oscillator relies on the slight change in shape of a quartz crystal under an electric field, a property known as inverse piezoelectricity. A voltage applied to the electrodes on the crystal causes it to change shape; when the voltage is removed, the crystal generates a small voltage as it elastically returns to its original shape.

As far as I'm aware, quartz oscillators work off inverse piezoelectricity, so an electric charge is applied, rather than mechanical stress.