Leisure hours among the gems

by Augustus C. Hamlin

Pliny entertained the same idea, and also that its infrangibility could be overcome only by first steeping it in goats’ blood. Even in mediæval times Ben Mansur, the Persian mineralogist, gravely states that a diamond laid upon the anvil and struck by a hammer would not be broken, but would be driven by the violence of the blow into the substance of the anvil. This stupid but wide-spread idea has prevailed even in modern times; and many a gem has been sacrificed by the ignorant in testing the character of the stone. The brittleness of the gem is partly due to its singular cleavage, which in regular crystals is so perfect and uniform as to permit the lapidary to remove the laminæ so as to entirely demolish the structure of the crystal. But when once accomplished, no artisan, however skilful, can replace them again. The facility with which the stone may be separated was known in ancient times among the Hindoos, and probably in Europe as early as the sixteenth century, as De Boot knew of a physician who could divide the diamond into thin scales like a piece of talc; but it was forgotten until Wollaston not many years ago stumbled upon the secret of cleavage and made it known to modern science.

The real charm and value of the diamond lie in its remarkable brilliancy, and in the wonderful prismatic display of the bright and beautiful colors, which are constantly fugitive, but perpetually returning, as the learned Abbé Haüy elegantly expresses it. When a ray of light is reflected from the surface of a body, a particular impression is conveyed to the eye, which we may properly term the eclat. This impression is often so decided and so varied in its effects, that we are able to distinguish certain substances at a glance; and the reflection from the diamond exhibits a peculiarity which is seen only in a very few substances. This is known as the adamantine flash, and none of the gems display it to any marked degree except the rare zircon. We witness the perfection of this property in the black and opaque but crystallized diamond, when faceted by art; and also in some few minerals of which we shall soon make mention. When the rays of light are refracted, after passing through the transparent diamond after it has been cut in a certain manner, and its facets are arranged in an exact relation to each other, then we obtain the remarkable exhibition of color which is known as the prismatic display. This singular property is seen in perfection, or even to any considerable degree, only in the diamond, among all the gems thus far known. But art, however, has succeeded in imitating it in one of her productions of glass, and so admirably, that under favorable circumstances it is quite impossible for the eye alone to distinguish the artificial from the real gem. Some of the theories relating to the causes of these phenomena we will discuss hereafter, and at the present will only say that it is to modern science the diamond owes the full development of its latent beauty; and that the result was not attained until Newton demonstrated the laws that govern the refraction of light. It is only in the brilliant and rose-cut forms, or their modifications, when made with mathematical precision, that the brilliancy and beauty of the stone is displayed in perfection. The ancients, therefore, were not acquainted with the full splendors of the gem. For, being ignorant of the laws of refraction of light, they polished the stone chiefly with the view of preserving its greatest weight; and, at the same time, producing perfect transparency. Hence most of the specimens of ancient and barbaric art are rudely cut, and therefore do not exhibit the degree of beauty which is latent in the mineral. This is also one of the reasons why the luxurious Romans preferred the opal to the diamond, since the polished, or even the rude specimens of opal exhibited their glorious reflections of wondrous hues, both by day and in artificial light by night; while the diamond, with its natural or polished faces, gave forth no prismatic display in the daylight, and but a slight degree comparatively in artificial light at night.

Whence arises this remarkable brilliancy, and to what particular cause is the property due? This inquiry has afforded a fruitful theme of speculation among philosophers, but at the present time we are content to say that the refractive power of the gem is due to the nature of its substance. This is somewhat indefinite, it is true, but what else can we say?

Under the general belief that the harder the gem the higher its refractive powers would be, it has been maintained that the brilliancy of the diamond arose from the simple property of its excessive hardness. Investigation, however, does not sustain this widespread view. Hardness, indeed, may have considerable relation with the arrangement and form of the molecules composing the gem, for in the same crystal it is not uniform,—some faces and angles being harder than others,—but it does not determine the degree of brilliancy. To strengthen this statement we will take for instances the soft minerals, crocroisite, the chromate of lead; the Greenockite, the sulphuret of cadmium; and the octahedrite, the oxide of titanium, which exceed even the diamond in brilliancy. There are also other decided examples among the transparent minerals to sustain this view; the most remarkable of which perhaps may be found in the zircon, a gem which is soft as quartz; yet it ranks next to the diamond in brightness, and far surpasses in eclat every other gem, even the sapphire, which is next to the diamond in hardness. Density does not seem to have anything to do with the determination of the refractive power of gems, for the garnet, spinel, sapphire, and zircon are much heavier than the diamond, and are yet far inferior in brilliancy. The topaz is exactly of the same specific gravity as the diamond (3.55), but nevertheless its refractive powers have but little more than one half the energy of the diamond. The relative brilliancy of the diamond to that of the purest limpid quartz is 8 to 3; but the relative density is only as 4 to 3. All diamonds do not exhibit the same degree of brilliancy, because they do not possess alike the same quality of purity or perfection of crystallization.

We often observe among the minerals that the most perfect specimens are found of a diminutive size; and we shall also find that the finest and purest types of the diamond occur in stones of little weight. The larger crystals, or amorphous masses, seem to be wanting in purity and brightness as compared with the lesser; and this peculiarity may be observed well marked in some of the other gems. Here, then, we may find material for the argument that the degree of brilliancy is in a measure due to the perfection of the crystallization of the stone; and, therefore, the larger and coarser the laminæ of the crystal the less will be its brightness. One thing, however, is certain; that the most brilliant gems are obtained from stones of no great weight, and which also seem, from their form, to indicate a nodular arrangement of particles in their formation; or, in other words, a certain concentric manner of crystallization. This form of deposition is not peculiar to the diamond, but is clearly shown in the sapphires, topazes, chrysoberyls, tourmalines; and the finest specimens of these gems are cut from these nodular forms. We think we are correct in stating that the greatest brilliancy and the most beautiful prismatic display may be observed in diamonds of less than ten karats in weight. In fact, the diminution of brilliancy in the gem, when above twenty karats, is easily discerned by the eye alone, as compared with the vivid and adamantine flash of a pure and perfect four or eight karat stone. The same peculiarity may be observed in the little globular masses of the chrysoberyl, which are seldom larger than a pea in size, but which, when cut, exhibit flashes of fire which are only equalled or excelled by the diamond, or the rarer zircon. We can hardly realize that the little rounded pebbles of white topaz, known as gouttes d’eau, “drops of water,” will yield gems of such lustre as to be often exhibited, and even sold for the diamond. Yet the larger irregular masses, or finely crystallized specimens of the same mineral, do not afford gems of unusual brilliancy. In these instances we may affirm that the form or mode of crystallization has something to do with the degree of brilliancy.

The prismatic play of color which this gem alone possesses to any considerable degree constitutes its chief charm, and its cause has been a matter of earnest study among opticians. A plausible theory has lately been advanced by an English philosopher that the colored rays are produced by the relation of the high refractive to its very low dispersive power. For instance, this refractive power in the diamond, or, in other words, its property of bending a ray of light falling obliquely upon its surface, is 2.439, while that of water is only 1.336, and that of glass 1.500. But its power of dispersing a ray of white light, or, in other words, of separating it into its compound colors in reference to its refractive power, is only 0.038, while that of glass is 0.052. Hence it is surmised that this inferiority of dispersive power is required for the production of the splendid colored reflections which constitute the glory of the gem. It is also maintained that this high refractive power separates the red and the blue rays more than a high dispersive power would in other transparent bodies, and to such degree as to allow each color of the spectrum its full force. As example, the zircon, with its inferior reflections, is offered, its refraction being 1.99 on the established scale, while its dispersive power is as high as 0.044. The relations of the spinel are also as 1.81 to 0.040, and neither does the gem display the rainbow hues. This theory is certainly ingenious, and if correct the test may be applied to other transparent minerals possessing similar relations. We may, therefore, expect the white garnet to exhibit the property of prismatic display, as it has a refractive power of 1.81 and a dispersive power of 0.033. But, unfortunately, perfectly pure and transparent white garnets are unknown, and we must therefore turn to other minerals for comparison.

To the white tourmaline, then, we will apply the test, since this mineral has a refractive power of 1.66, with a dispersive power of only 0.028. Here, then, we have nearly the same relation as observed in the diamond; and, if the theory be correct, we may reasonably expect the exhibition of the same phenomena. But, upon examination of several perfectly white and transparent tourmalines from Mt. Mica, cut into regular brilliants, we have failed to detect an increase of prismatic display, or even discover any evidence to lend support to the plausibility of the hypothesis. We, therefore, reluctantly turn to other arguments for a solution of this most interesting problem.

The snow-white diamond displays the rainbow hues in the greatest perfection; and this is the reason why this quality is sought for in preference to the light buff or deeper yellow, which are in reality more brilliant. The deeper the hue of the gem, the less becomes the prismatic display; and when the diamond becomes of deep and decided hue, the colored reflections cease altogether. It is somewhat singular that the colored gems are generally more brilliant than the pure white, that is, if the color is not so deep as to affect the transparency of the stone. For examples, we shall find that the white sapphire has an index of refraction equal to 1.768, while the blue has 1.794, and the red 1.779. The refractive of the white topaz is 1.610, while the yellow is 1.632.

The brilliancy and rainbow play of the diamond is not so apparent by daylight as by certain kinds of artificial light, when all its latent beauties are called forth as if by magic. The light of the camp-fire in the obscurity of night produces a marvellous effect upon the polished stone; and it is no wonder that the savage heart of the Russian General, Suvaroff, was fascinated by the vivid gleams of his treasured diamonds when viewed at night in the flickering beams of his bivouac fire. It may seem singular that the brilliant white light of gas does not display the qualities of the diamond as the duller flame of the wax candle. The secret lies, perhaps, in the difference in their spectra. Nevertheless, there is a great difference in their effects upon the gem, and it is a fact that the wax candle far exceeds the gaslight in calling forth the latent splendors of the gem. Therefore, we can assert that the brilliancy of toilets where the diamond is much worn depends greatly upon the manner of illuminating the apartment.

We now come to another interesting problem in the study of the nature of the diamond. We refer to the various colors of the gem. As we have maintained that the mineral is of vegetable origin we may be expected to explain the phenomena of its color upon this hypothesis, and also account for the various changes of the gem when exposed to the effects of heat or the fire test. But we must admit with candor that our views concerning this physical property are decidedly unsatisfactory, and shall refer the reader to one of the chapters in our treatise on the Tourmaline, in which are grouped some of the theories relating to the subject. In fact, we may repeat the remarks of Huyghens, who said at the end of the seventeenth century: “In spite of the labors of Newton, no one has yet fully discovered the cause of the color of bodies.” “We must, then,” says M. Babinet, “admire, without penetrating their secret, the unparalleled red of the Oriental ruby, the pure yellow of the topaz, the unmingled greenness of the emerald, the soft blue of the sapphire, and the rich violet of the amethyst. This is not the only thing the discovery of which we shall leave to posterity.”