Why Piezoelectricity is a two-way (reciprocal) electromechanical effect found in certain crystals such as quartz, Rochelle salt and various ceramics: mechanical stress on the crystal separates charge and produces a voltage across it, and conversely an applied voltage physically deforms the crystal. That reciprocity is what makes a quartz crystal act as a very high-Q mechanical resonator in oscillators and filters, and it is also the basis of crystal microphones, phonograph cartridges, buzzers and ultrasonic transducers.
Watch out The choice about generating a voltage when an electromagnetic wave hits the material describes the photoelectric or photovoltaic effect, not a mechanical one; the index-of-refraction answer describes birefringence, an optical property.
Piezo = pressure. Squeeze it and you get volts; apply volts and it squirms. Mechanical in, electrical out, both ways.