These naturally occuring minerals demonstrate piezoelectricity when struck together or rubbed together briskly. Just turn down the lights and watch the sparks fly!
The piezoelectric effect is the quantum process where a material subjected to an electric voltage will change size or move. Alternately, the same material will generate an electric voltage when stressed or deformed. The piezoelectric effect is used for batteryless electric ignition of gas heaters and lighters, and lets us keep accurate time with quartz watches.