Exactly. Large crystal deposits are piezoelectric. I actually did a research project on it once. Stresses in the Earth can cause large crystal deposits (or any quartz crystal containing rocks) to discharge, which can be anywhere from tiny and only detectable by equipment, to enormous enough to ionize the air. HOWEVER, they do not continuosly oscillate, it's more like a piezo sparker. The process can even be reversed, by pumping huge discharges into these rocks, they can acually force faults to move.