Leisure hours among the gems

by Augustus C. Hamlin

With the aid of the tourmaline tongs, however, we have an easier method—subject to a few exceptions—of detecting the properties of refraction of all gems even when they have been cut. The transparent tourmaline, when cut in thin plates parallel to the axes of its natural crystals, possesses the strange and extraordinary power of extinguishing or causing to disappear one of the rays of polarized light, while the other is preserved. Therefore, when a body possessing single refraction, although perfectly transparent, is placed between the two thin slices of tourmaline composing the tourmaline tongs or polariscope, no light passes through; the instrument does not afford the least glimmer of a ray of light. But when the transparent body interposed in the polariscope is of double refraction, light passes through as if by magic.

When the gem has been cut for ornamental purposes it is often difficult to adjust the faces of it to the plates of the polariscope. Then recourse may be had to the use of a glass cell containing a fluid of a high refractive power, like the oil of cassia, turpentine, sulphuret of carbon, etc. The stone immersed in these fluids admits the light in all directions, and is then easily viewed through the plates of the instrument. These fluids, however, are not of sufficiently high refractive powers to do justice to the diamond and the zircon, but they answer admirably for all gems of a less refractive power, like the varieties of sapphire, spinel, topaz, garnet, tourmaline, emerald, etc.

The tourmaline tongs furnish the expert with a rapid and easy method of detecting many of the gems by reason of the phenomena of their refraction. But the experimentalist must be on his guard lest he pronounce substances to be of double when they really possess but single refraction. Glass has a tendency to crystalline regularity when heated and cooled suddenly, and may therefore acquire the property of polarizing the ray that passes the first plate of tourmaline and dispose of a part of that which passes the second. Certain minerals of the cubic system produce the same result by reason of a certain rare but forced arrangement; and some crystals, like the topaz, when cut in a certain direction to their optical axes cease to exhibit the phenomena of double refraction.

One of the chief tests used by the jewellers of olden times to distinguish the diamond was the test of the tincture. This tincture was a varnish made of ivory black and mastic, and when applied to the back of the diamond seemed to increase its lustre, while other gems were impaired in their natural effects. Modern investigators, however, have shown that this procedure is a fallacious one, and is in reality one of the absurd traditions which have been attached to the gem from early times, like some of the supposed spiritual properties.

A perfect diamond must stand the tests for purity, faultlessness, and transparency, and when these are carefully applied to the stone perfect gems will be found to be very rare.

As we have said before, the diamond is the foulest of gems, and is exceedingly liable to be injured by faults, such as are described in the technical terms of the jewellers as ashes, gray spots, rusty places, flaws, cavities, fissures, veins, feathers, foreign bodies, wavy and vitreous spots. Very few diamonds can stand the test of the microscope and be pronounced perfect. Still these microscopic faults are not to be considered in the commerce of the gem, but only in the study of its origin and nature. The jeweller may properly pronounce perfect the gem whose faults cannot be detected by the human eye. Even the magnificent Regent has one small foul speck in it, according to Jeffries; and Sir David Brewster found in the Koh-i-noor three specks, or rather cavities, in its central portion, which appeared to view in more or less distinctness according as the light reached them.

After a diamond has been cut it is not easy to ascertain its degree of perfection without careful examination, and this occupies considerable time. And gems which appear at first glance to be pure and perfect are often found to be defective after examination.

Babinet, of the French Institute, adopted the following method to study the effects of the diamond, and it was his intention to apply the test during his leisure moments to the principal diamonds in France; but other important labors diverted him from his purpose. He pierced a hole in a white card, a little larger than the diamond to be examined, and then passed a ray of sunlight or of the electric lamp through this hole. In the pathway of this ray, at a certain distance from the hole behind the card, he placed the diamond so that the ray of light fell upon the anterior surface of the stone. The rays reflected from this anterior surface, or, in other words, the table of the diamond, and those which pass through the stone are reflected back on the card, where they exhibit a white image of the table surrounded by small bands iridescent with the prismatic colors. By this simple method Babinet found that if the diamond had been well cut the colors were considerable in number, were well separated, and equally spread around the white reflection of the table. As each of these bands indicates one of the lustres of the stone, it is easy to estimate them both in number, quality, and symmetry. Therefore the observer can not only detect the errors of the cutting of the gem, but decide upon the form best adapted for the stone.

The term used to express the weight of the diamond and all the gems is derived from the word Keration, a kind of vetch, whose seeds, being generally of a uniform weight, furnished the Orientals with the means of estimating the value of precious stones. It is supposed to represent the equivalent of four Troy grains, but by actual measurement the diamond karat weighs but 3¹⁄₃ Troy grains at the present day, and it may descend even lower in the scale, unless the unit be established by law. The history of the series of diminutions by which the karat has reached its present weight is obscure, but as the term is supposed to represent four Troy grains it should equal them in reality. That it is a mere conventional weight is shown by its variance in European countries, as well as in the gem-producing countries of Asia. In making use of the term to express the weight of precious stones we would suggest that it be written karat, as more in accordance with its derivation, and that the commonly accepted word carat be used when we wish to define the alloy of certain metals, like that of gold and silver coins.

Before proceeding to the subject of the valuation of diamonds, we will say a few words concerning the imitations produced by the skill of man. Many attempts at imitating the diamond have been made by experimentalists for a long time past, and much ingenuity shown by them. To those of our readers who desire to study especially these experiments we will refer them to the works of Silliman, Hare, Latour, Saix, Despretz, Dumas, Ebleman and Gaudin, Mohler, St. Clair, Deville, Gaunal, Becquerel, Joyce, Cagnard de la Tour, Mactear, Hannay, and many other well-known experimenters.