Diamonds And Luxury

Many precious stones when exposed to certain external influences acquire a greater or less charge of electricity. They differ from each other in the length of time this charge can be retained, some retaining it for a considerable time, others for a less time, perhaps only a few minutes.
The French abbe, Haiiy, the founder of modern scientific mineralogy, attempted to make extensive use of these characters as a means of identifying stones and distinguishing them one from another. In his book, published in 1817, Traitfc des caracteres physiques des pierres precimses, he devoted seventy-two out of a total of two hundred and fifty-three pages to the consideration of electrical characters, while the optical characters are dismissed in thirty-two pages. A comparison with the number of pages devoted to the treatment of these two branches in the present volume, shows how much more important to-day is the consideration of the optical characters of minerals.

The examination of the electrical, as of the optical, characters of a stone, has the advantage that no injury to the stone results therefrom. The observation of electrical characters, however, requires a certain amount of skill and practice; for the detection of the very small electrical charges acquired by most precious stones is difficult; and, further, these observations must be conducted in a perfectly dry atmosphere, a condition not always easy to obtain. Any charge located on the surface of a stone is rapidly lost in a damp atmosphere, and a stone which retains its charge in dry surroundings will rapidly lose it in the presence of moisture. The length of time a stone retains its charge, a test to which Haiiy attached great importance, depends largely therefore upon external conditions.
At the time Haiiy was engaged on his researches the methods of electrical investiga¬tion were, at least for his purposes, fairly well developed, while methods for the optical investigation of minerals had received little or no attention. Check also egl.Observers had indeed noticed that some minerals were singly refracting and others doubly refracting, but there was no polariscope to give precision to their observations and the phenomenon of dichroism had yet to be discovered. We can thus readily understand why Haiiy attached so much more importance to the electrical than to the optical characters of minerals and precious stones. With the discovery of the dichroscope and a convenient polariscope the optical characters of minerals assumed their true importance, while their electrical characters became a minor consideration, as may be gathered from their brief mention in this place.

For the purpose of demonstrating the existence of a charge of electricity upon the surface of a stone an instrument known as an electroscope may be used; for very feeble charges an electrometer of complicated construction will be necessary. Haiiy employed for this purpose an " electrical needle" ; it consisted simply of a brass rod, terminated at either end by a small brass ball, and balanced on a vertical fine steel point, on which it could turn freely like a magnetic needle. An electrically charged body, when presented to either of the balls, would attract it. By giving an electric charge to the balls, they would be attracted or repelled on the approach of a body according as its charge was unlike or like that of the balls. The electric pendulum, consisting of a pith ball suspended by a silk thread, served the same purpose. With the help of such instruments it is easy to demonstrate that minerals, including precious stones, become, under various conditions, charged with electricity ; the fact of itself is, however, of little note.

After rubbing on cloth all precious stones, like glass, become positively electrified. Topaz and tourmaline become strongly electrified after such treatment, diamond less strongly, and the majority of precious stones only feebly. Smooth faces are more susceptible of electrification than are rough ones, and hence cut stones furnish the most favourable material for this purpose. In perfectly dry air, some precious stones retain an electrical charge for a comparatively long period ; this is especially so in the case of topaz, the electrification of which can be detected after an interval of thirty-two hours ; sapphire will retain its charge for from five to six hours, and diamond for half an hour. Colourless topaz, colourless sapphire, and diamond may be distinguished by this difference in their behaviour ; after imparting a charge by rubbing with a cloth, the stones should be laid on a metal plate and their electrical state tested from time to time. The majority of precious stones lose their charges with great rapidity, some, indeed, after only a few moments.

Amber, like other resinous substances, becomes negatively electrified on rubbing, and so strongly that it attracts to itself any light bodies, such as pieces of paper. These, after contact with the amber, themselves become charged and are then repelled by it. This particular character of amber is of value as a means whereby it may be distinguished from its imitations, which will be mentioned later under the special descriptions.

Dont forget this diamond engagement ring.The electricity developed on some precious stones when under the influence of changes of temperature is known as pyroelectricity. The charge produced in this way on the surface of a stone varies in sign at different areas of the surface, the charge at one point being positive while that at another is negative. Those parts of the surface which become positively electrified on heating become negatively electrified on cooling, and vice versd. Tourmaline and topaz are remarkable for the strength of the pyroelectrical charge they acquire; and this distinguishes them from other precious stones, which when exposed to the same influences acquire but feeble charges, or none at all. Thus, with the help of one of the electrical instruments mentioned above, a red tourmaline can be distinguished from a ruby, and a greenish-blue topaz from an aquamarine of the same colour; for the former in each case will show a strong pyroelectrical charge, and the latter none. During the gradual cooling of tourmaline after being heated, it assumes the power of attracting light bodies to itself, as does amber after being rubbed.