When a quartz crystal is deformed by mechanical stress in a specific direction, polarization occurs within it, and charges of opposite polarity appear on its surface. Once the external force is removed, the crystal returns to its neutral state. However, when the direction of the external force changes, the polarity of the charges also changes. This phenomenon is known as the piezoelectric effect of quartz crystal.
Conversely, when a quartz crystal is subjected to an alternating electric field, it undergoes mechanical deformation in a specific direction, known as mechanical vibration. Once the applied electric field disappears, this deformation disappears. This phenomenon is known as the inverse piezoelectric effect, also commonly referred to as the electrostrictive effect.
Crystals that exhibit the piezoelectric effect are called piezoelectric crystals, and quartz crystals are a prime example. In fact, the piezoelectric effect was first discovered in quartz crystals.