The most important of the physical properties possessed by precious stones are those of light, for to these all known gems owe their beauty, if not actual fascination. When light strikes a polished stone, the following effects are observed:
(2) reflected; or produce
The third physical property of light, Phosphorescence, is the property possessed by certain gems and minerals of becoming phosphorescent on being rubbed, or on having their temperature raised by this or other means.
It is difficult to say exactly whether this is due to the heat, the friction, or to electricity.
Perhaps two or all of these may be the cause, for electricity is developed in some gems—such as the topaz—by heat, and heat by electricity, and phosphorescence developed by both.
For example, if we rub together some pulverised fluorspar in the dark, or raise its temperature by the direct application of heat, such as from a hot or warm iron, or a heated wire, we at once obtain excellent phosphorescence.
Common quartz, rubbed against a second piece of the same quartz in the dark, becomes highly phosphorescent.
Certain gems, also, when merely exposed to light—sunlight for preference—then taken into a darkened room, will glow for a short time.
The diamond is one of the best examples of this kind of phosphorescence, for if exposed to sunlight for a while, then covered and rapidly taken into black darkness, it will emit a curious phosphorescent glow for from one to ten seconds; the purer the stone, the longer, clearer and brighter the result.