Main Source: The Piezo Dielectric Properties of Crystals and Gems, A Quantum Physics Paper by Sharon Whale, http://whalemedical.com/the-piezo-dielectric-properties-of-crystals-gems/
Quartz responds to a.) piezo - mechanical pressure, b.) electric frequencies, c.) electro-magnetic frequencies, and d.) possibly other unknown forces.
Piezo means pressure (Greek). Quartz crystals take mechanical pressure frequencies and transform them into electrical frequencies. These electrical frequencies can be measured.
...In the early electrical gramophones the P.E. crystal acted as a link in sound reproduction via the needle, the quartz was pressurised by the music etchings in the record's grooves....
[The Piezo Dielectric Properties of Crystals and Gems]
Quart take piezo mechanical frequency and converts it to electrical frequencies. Quartz also functions in the opposite direction. Quartz takes electrical current (frequency) and converts it to mechanical vibration (frequency). For example, in radios, quartz will vibrate in response to electric current near it.
...when alternating electrical voltages are applied to (or near) a quartz crystal, the crystal will mechanically vibrate. The Piezo Electrical (P.E.) effect of quartz crystal is a scientific principle, used in television, radio and medical equipment, such as ultra sound diagnostic apparatus and ultra sonic therapies....
[The Piezo Dielectric Properties of Crystals and Gems]
...Liberating the electrical charges when the crystal is stressed is a function of electrical asymmetry within the atomic groups of which the crystal is built...
[The Piezo Dielectric Properties of Crystals and Gems] In the unstressed quartz atom, the positive and negative charges are in an electrically neutral arrangement. However, when the quartz atom is mechanically stressed by pressure, electrons are displaced. This is what causes the electrical force or field.
...Quartz is also pyro electric and additionally, it will resonate when influenced by electromagnetic fields....
[The Piezo Dielectric Properties of Crystals and Gems]