In certain crystals =quartz that have polar molecules - quartz, topaz and tourmaline - a mechanical; stress applied to the crystal produces polarisation of the molecules. this is known as the p[piezoelectric effect. The polarisation causes a potential difference across the crystal, and this in turn can be used to produce a current. This effect can be used in transducers - a transducer is a device that changes a mechanical into an electrical signal e.g. a microphone). The converse piezoelectric effect works to change a voltage into a mechanical signal is used in loudspeakers and watches. The mechanical signal produced is at the natural frequency of the quartz which makes it very suitable for watches.