Mineral Guide > Gems > Electrical Properties of Gems

Electrical Properties of Gems

All gems when rubbed upon cloth become, like glass, positively electrified. Gems differ, however, in the length of time during which they will retain an electrical charge. Thus tourmaline and topaz remain electric under favorable conditions for several hours; but diamond loses its electricity within half an hour.

The electrical peculiarities of different species were at one time used quite extensively for identifying them ; but owing to different behavior under different atmospheric conditions little use is now made of such tests. Besides developing electricity by friction some gems become electric upon heating. Such are said to be pyro-electric. To test a stone, or rough piece of mineral for this property, it can conveniently be held in forceps and heated gently in a colorless flame, such as that of a Bunsen burner or alcohol lamp. The amount of heating should not be much over 100 degrees C. On withdrawing the stone, it will, as it cools, if pyroelectric, attract bits of tissue paper or straws.

Tourmaline is perhaps the most strongly pyro-electric of the minerals used as gems, and the property affords a means for identifying it. Topaz is another gem mineral which usually exhibits this property. Some topaz also becomes electric when subjected to simple pressure. This is said to be true of some crystals of Brazilian topaz if they are pressed between the fingers in the direction of the vertical axis. Electricity developed in this way is known as piezo-electricity.

Simple tests for all these kinds of electricity consist in the attraction of light objects, such as bits of tissue paper, cat hairs, pith balls suspended by silk threads, etc. They are best made when the atmosphere is dry, the winter season being especially favorable.