Electronics/Crystals
The vast majority of crystals used in electronics are piezoelectric crystals.
The piezoelectric effect (converting back and forth between electrical voltage and mechanical position) makes quartz crystals physically vibrate. The quartz crystal can be cut into the right shape and appropriately mounted so that it "rings" (vibrates) only at one precise frequency, useful for clock circuits.
Some quartz crystals are used as SAW filters.
PZT crystals are used in ultrasonic devices such as Medical ultrasonography. They use the piezoelectric effect to convert electrical energy to sound energy to send out a pulse, then convert the sound energy of the echo back to electrical energy.
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(The atomic clocks in GPS satellites use the oscillations of cesium atoms in gas form as the time reference. That gives far better precision than any known crystal oscillator.)