Piezoelectricity is an interesting physical property that can be used to make very good microphones, sensors, transducers, pickups and other transducers. Most piezoelectric devices sold today use hard materials known as piezoceramics, which as the name implies, are composed of relatively hard, ceramic or glass-like materials. That's what you see on the left: Many circular piezo discs mounted on a plate. These particular discs are used to make ultrasound imaging devices.
But there are many other types of piezo materials. There are plastics called PVDF that have piezo properties, and there are inadvertent piezo materials, such as the back plastic inserts in some cheap XLR connectors.
We found this out by accident when we dropped the cable end on the hard cement floor, and heard the bang through the headphones, without a microphone attached! The "device" we discovered was nothing more than than the piece of plastic between the pins vibrating and creating a voltage, and not a small one! This "no microphone, microphone" could be used in high intensity applications, like monitoring blasts of explosives. Cheap too.
Even the dielectric in coaxial and microphone cables, when hit or bent, exhibits a piezoelectric effect, which can produce unwanted noise. Good cable makers know this and avoid certain materials that are more prone to piezo noisemaking.
Try it yourself. Get a cheapo cable from RS, connect the proper end to your hi gain preamp and nothing to the other end and put on the headphones. Tap it and listen.