Electrical Properties

Gems may have a number of other interesting characteristics unrelated to their appearance or durability. Among these, electrical behavior is sometimes remarkable. Museum curators and jewelers have long known that their tourmaline specimens and gems will accumulate thick coats of fine dust in a short period of time even when they are displayed in tightly sealed showcases. As the case lights are turned on and off each day the tourmaline is alternately heated and cooled. When heated, tourmaline develops a substantial electric charge which quickly attracts tiny dust particles in the air. Before long, a gem will be coated. This characteristic is known as pyroelectricity—electricity produced by heating. Diamond, topaz, tourmaline, and amber, when polished briskly with a cloth, will even develop enough of an electric charge to attract and hold small bits of paper.
An electrical effect discovered by Pierre and Jacques Curie in France in 1880 is perhaps even more remarkable. These two physicists, studying the ability of crystals to conduct electricity, found that when certain crystals were squeezed they developed a measurable electrical charge. The phenomenon was later called "piezoelectricity," based on a Greek word piezin, meaning "to press." As early as 1881 another Frenchman, G. Lippman, predicted that a reverse phenomenon would take place. He suggested that an electric charge placed on any piezoelectric crystal would cause it to change shape. This was successfully tested by the Curie brothers. Today this important characteristic is applied, using quartz and other piezoelectric substances, to control the frequencies of all radio broadcasting and other electronic devices. The alternating current in these devices and the piezoelectric crystal plates inserted in them must operate in unison. Thus, the fixed dimensions and structure of the plate keep the current alternating only at the rate at which the crystal plate can change its shape. Such a device is known as a frequency control oscillator. It is responsible for the fact that every time you tune in your favorite radio or television station it is at the same number on the dial, just where it was last time.