The testing of the diamond is generally an easy matter to the expert, but the merchant and the amateur are often unable to decide quickly concerning the character of a gem that may be offered to them. The learned Abbé Haüy was often called in consultation upon matters of this kind, and even appeared in court as an expert of diamonds. Fortunately, the methods of ascertaining the diamond beyond a doubt are now so easy and numerous that few attempts are made like those of the last century, when white topazes and zircons were sold or substituted for diamonds.
We may be permitted to say that the first and surest test of the gem is that of hardness, and that the diamond scratches all other substances with ease. The application of this test is not always made with facility, but a little practice with a rough crystal of diamond will soon enable the experimenter to perform it satisfactorily. In performing the test for hardness for all gems it is well to be provided with a fine English file, and splinters of quartz, topaz, and sapphire. The file, adroitly applied, will detect the character of gems below the quartz in hardness, and the splinters above mentioned will determine all others but the diamond.
In applying these tests, it is well to touch the middle of a facet, if the gem be polished, and remember that the edges of many of the gems are very brittle. The diamond, for instance, although it is so very hard when its surface is directly tested, yet its thin edges, when cut into a gem, are broken down with the greatest ease; therefore the test of hardness should be applied to the polished face of the stone.
But when the stone has been polished and set in such a manner that the test of hardness cannot be easily applied, then the easiest method of distinguishing its character is to expose it to the rays of artificial light and observe their effects. If the stone, when placed at a certain distance from the candle, flashes out the prismatic hues in a marked manner, then the observer may safely conclude that the object is either a diamond or one of the artificial gems known as strass. To distinguish them by means of the eye alone will be a difficult task, for both possess very high refractive powers and exhibit remarkable displays of the prismatic hues; but the application of a fragment of diamond, sapphire, or topaz to the object will at once make known its character, and even the test of the steel file will betray the softness of the glass.
The transparent zircons closely approach the diamond in brilliancy, and are among the rarest of gems; in fact, a fine specimen of perfect purity would be worth more to the amateur than a diamond of equal weight. The pure yellow are declared by King to be among the rarest and most beautiful of gems on account of their remarkable brilliancy and lustre. He also states that they are seldom found above two karats in weight.
The white zircon, which resembles the diamond so closely, is always minute and rarely above one karat in size. All that are known in commerce come from Matura, in Ceylon, and are sometimes called Matura diamonds. In times past they were in great demand for incrustation work of watches and jewelry, for when cut in the form of roses and set upon brilliant foil they could not be easily detected by the eye from the diamond. At the present day they are seldom seen in commerce, but whether from scarcity, or from an improvement in the ideas of honor in the jewellers, we are unable to state. We know, however, that they are still mined by the natives of Ceylon, and are used in Asia for ornamental purposes. The white spinel is sometimes mistaken for the adamas; but its inferiority in hardness and its want of the prismatic hues render its detection easy. The white topaz, when of two or three karats weight, resembles the diamond in the rough very closely, especially when in the modified or globular forms of crystallization. The degree of specific gravity is exactly the same; but the test of hardness and the want of the prismatic display serve to dispel all doubts as to its nature. Neither of these gems, nor the white sapphire, has that peculiar sharp flash of the diamond which is known as the adamantine eclat. It has been stated by Hoppè that some of the Brazilian diamonds do not exhibit any more play of color when cut than rock crystal; but we have no doubt but that the writer is in error, and has probably been deceived by some of the white spinels or topazes which are sometimes found in the diamond mines of Brazil and are of great brilliancy.
To estimate the value of a rough diamond and ascertain its purity is often a difficult task, and one that requires both education and tact. For the surface of the natural gem, especially the modified and spheroidal crystals, is generally dull and chatoyant. This singular appearance, which has been erroneously represented as a thin crust, is in reality produced by the salient edges of the laminæ of which the stone is composed. The same or a similar effect may be artificially produced in the polished diamond by heating it to a white heat, as has been seen in the diamonds injured in the great fires of Hamburg and Chicago.
Barbot, the French jeweller, declared that he had discovered a means by which the apparent rough, translucent coating could be changed into perfect transparency so as to give a clear view of the condition of the interior. This statement, however, is very much doubted by lapidaries.
When the rough diamond is placed in turpentine, alcohol, or even water, it appears more transparent than before and like a bubble of air. We have therefore sometimes thought that Barbot had really discovered some fluid of high refractive power, immersed in which the rough diamond became transparent. For it is a well-established fact known among chemists, that rough gems of a lower refractive power, like sapphire, chrysoberyl, spinel, etc., if placed in a fluid possessing an equal refractive energy, like that of muriate of antimony, become clear, and the observer is enabled to look through them. This discovery strengthens Barbot’s statement and places it within the possibilities; but as yet we know of no fluid that possesses a refractive power equal to the diamond. Topaz, emerald, and other gems of low refractive power can be readily examined by using the oil of cassia, which has the refractive energy of 1.64.
The rough diamonds often exhibit various colors, but generally of faint tints. Sometimes the hue is not perfectly distributed throughout the stone, but may be confined to a part of it, as in the sapphire, or even in one of its laminæ. It is a singular fact that the external coverings of the mineral often contain the shades which render the rough gem of a disagreeable hue, and which may be made to disappear by the process of cutting. Therefore, it sometimes requires experience and tact to determine the value of a rough diamond which possesses a decided hue. However, those stones which exhibit a greenish or reddish color are considered safe stones for investment, and will develop into superior gems after cutting. The bluish and the blackish are thought to be harder than the others; while the yellowish and grayish colors indicate inferior stones. The brownish and yellowish hues may disappear in process of cutting, but there is greater uncertainty in these shades than in the others. Sometimes a stone which promises to yield a perfectly white gem, after cutting turns out to be of a decided disagreeable tinge. Therefore the cutting of a rough diamond is often attended with some hazard.
The Hindoos have a practice of examining their stones before purchase by placing them in an aperture in the wall, with a lighted lamp in the rear. And to ascertain their color they take them under the cover of a tree thick with foliage, where the verdure of the shade quickly reveals any other tinge, especially the bluish. It is quite difficult to distinguish the snow-white, except by contrast. The officers of the Junta Diamontina, in Brazil, have a rough way of testing the native diamonds from quartz, white topaz, and spinel, by rubbing them together close to the ear. The sharp tone which is thus produced is characteristic, but the test requires a keen and practised ear.
Specific gravity is one of the best tests for determining the nature of polished stones. By this means some of the precious minerals can be detected with ease, like the zircon and the emerald, notwithstanding their color. But when applied to the diamond the experimenter should remember that its weight is exactly that of the white topaz; yet to distinguish them, the degree of hardness or the phenomena of electricity will at once enable the expert to discriminate with ease and without a doubt. For the best methods of ascertaining the specific gravity of the stones, we must refer the reader to elementary works on chemistry and physics.
The test of its electrical phenomena is an important one, as one can thereby almost instantly detect it from other gems, especially the white topaz. When rubbed it exhibits vitreous electricity like glass, but loses it in a very few moments.
Another curious phenomenon, called single refraction, enables the observer to distinguish the mineral from all other gems except the garnet and spinel, the others having double refraction, or, in other words, giving a double image of a candle-light when it is viewed through their facets.
For the purpose of observing this phenomenon, Sir David Brewster invented an instrument which he called a lithoscope. It consisted of a small glass prism which moved around a fixed joint so that the lower surface of it could be laid upon the surface, or a facet of the stone to be examined. In this position, the two surfaces being parallel, the image reflected from the lower surface of the prism would coincide with that reflected from the surface of the stone. A drop of the oil of cassia or of sulphuret of carbon is placed between the prism and the facet, and then the observer turns a screw to raise the prism a little round its joint. The effect of this is to separate the image of a light or a small luminous aperture as given by the prism from that given by the facet; and the difference in the intensity and the color of these two images is an infallible indication of the nature of the stone. The image from the diamond will be many times brighter than that reflected from the face of the prism when testing any of the other precious stones.
A simpler mode is sometimes adopted by experts, but it requires some dexterity to exhibit the property. The method is this: the diamond is held up to the eye, and a needle point or a small hole pierced in a card is looked at. If the object is seen double, as if there were two needle points or two holes, then the stone examined is not a diamond, as but one aperture should be seen.
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.
Despretz’s experiments, which were based upon certain combinations of carbon, are deserving of mention. The chemists have discovered that in combining sulphur and carbon a colorless liquid is produced resembling water, and apparently containing nothing but sulphur and carbon. Therefore, Despretz reasoned, if he could get rid of the sulphur by some manner, the carbon might be crystallized. And to obtain this result, the action of the volcanic battery offered the most plausible means. With the aid of this battery the experimenter really succeeded in obtaining on a thread of platina, passed through a solution containing carbon, some small crystalline depositions, which by their form and hardness seemed to be embryonic diamonds. But here the experiment ended. Nature refused to reveal her secrets.
The alchemists of the Middle Ages seeking the transformation of gold from baser metals, have been well represented by the chemists of the present century attempting to imitate the diamond. Philosophy and science have united their efforts in these fascinating experiments; but Nature still defies the most determined efforts of art in respect to the reproduction of the diamond.
The mineral appears to be an allotropic form of a simple elementary body which Nature offers to us with lavish hand. And when we consider the triumphs of chemistry, the process of transforming this element into the coveted form does not seem to be so very difficult to the casual thinker. But Nature is stubborn in revealing her simple acts. However, we are not without faith in these determined efforts of scientific skill; for we know that art now produces the brother of the diamond, graphite, at will. And we see that at the soda works at Aussig this form of carbon is obtained as a secondary product by the decomposition of cyanogen and its combinations. We do not, however, look forward with much pleasure to the realization of this idea; for success in producing the diamond will annihilate at a single blow an important article of commerce, and rob ornamentation and investment of one of its most desired objects.
Art, however, in its researches on this subject, has succeeded in producing a glass which, when cut, approaches very closely the brilliancy and prismatic display of the diamond. The artificial gems made from this glass, which is supposed to have thallium as a base, instead of lead, are really superior examples of art. “Nothing but glass,” is a phrase too often used contemptuously and unjustly; for these imitations are quite as charming as the adamas itself. In the flash of their rainbow hues, they surpass some of the great diamonds, like the Koh-i-noor; and in brilliancy they exceed all other gems. Their refraction reaches 2 on the established scale, while that of the diamond is reckoned at 2.4, and that of the sapphire, 1.79. They lack, however, hardness; and the effect of time dims their lustre. But this defect may yet be remedied; for the ancients made glass quite as hard as quartz. And when we come to examine those wonderful specimens of ancient glass, with their exquisite colorings, exhumed by General Di Cesnola in the Phœnician tombs of Cyprus, who will venture to establish a limit to the art of glass-making? Even within the past few years, a process has been discovered by which the elasticity and hardness of glass have been increased to a remarkable degree; and if the defect of brittleness can be overcome, a new era in glass-making will have been reached.
Artificial diamonds are often worn at the present day; and the fair wearer consoles herself with the hope that, when sunnier days come, the artificial will give place to the real. It will not be soon forgotten by the votaries of fashion, that the Duchesse de Berri, arriving in France, received for her bridal ornaments only the imitation, and that she wore them.