What is piezoelectric materials?

Piezoelectric effect is a property of some crystals (for example, quartz) to develop an electromotive force or voltage across opposite faces when subjected to a mechanical strains, and conversely, to expand or contract in size when subjected to an electromotive force. Piezoelectric crystal oscillators are used as frequency standards (for example, replacing balance wheels in watches), and for producing ultrasound. The crystals are also used in gramophone pickups, transducers in ultrasonics, and certain gas lighters.