Trade secrets... Piezoelectric crystals

When a piezoelectric crystal is bent or stressed the distribution of charge inside the crystal changes. This results in the creation of positive and negative charges on opposite faces of the crystal, generating a voltage. If we connect wires to the faces of the crystal we can make use of this voltage. Examples of piezoelectric materials include quartz, Rochelle salts (potassium sodium tartrate), many plastics, table sugar and even bone.