Piezoelectricity has the function of certain crystals to generate a voltage in response to applied mechanical stress.
The result is reversible in that the piezoelectric crystals, subject to an externally applied voltage, can change shape by a minimal amount.
The change is in the degree of nanometers although there are useful applications; such as, the production and detection of sound, the generation of high voltages, electronic frequency generation, and the ultrafine focusing of optical assemblies.
A characteristic known as pyroelectricity, which is the ability of certain mineral crystals to generate electrical charges when heated, was determined as early as the 18th century, and was named by David Brewster in 1824.
In 1880, the brothers Pierre Curie and Jacques Curie predicted and demonstrated piezoelectricity using tinfoil, glue, wire, magnets, and a jeweler's saw.
They showed that crystals of tourmaline, quartz, topaz, cane sugar, and Rochelle salt (sodium potassium tartrate tetrahydrate) generate electrical polarization from mechanical stress.
Quartz and Rochelle salt exhibited the most piezoelectricity. There are twenty known natural crystal classes that exhibit direct piezoelectricity.