Zero temperature coefficient resonators can be made when the plates are cut along the proper directions with respect to the crystallographic axes of quartz. It is abundant in nature. It is easy to grow in large quantities, at low cost, and with relatively high purity and perfection. The direct piezoelectric effect was discovered by the Curie brothers in 1880. The strains reversed when the voltage was reversed.
Piezoelectric crystals lack a center of symmetry.
When a force deforms the lattice, the centers of gravity of the positive and negative charges in the crystal can be separated so as to produce surface charges. The piezoelectric effect can provide a coupling between an electrical circuit and the mechanical properties of a crystal. Quartz crystals are highly anisotropic, that is, the properties vary greatly with crystallographic direction. The constants of quartz, such as the thermal expansion coefficient and the temperature coefficients of the elastic constants, also vary with direction. That crystal units can have zero temperature coefficients of frequency is a consequence of the temperature coefficients of the elastic constants ranging from negative to positive values.