Leisure hours among the gems

Before explaining, or rather attempting to explain, the phenomena of some of the prominent physical properties of the mineral, it is proper that we should give a description of its forms and its natural appearance as it is taken from the mines, so that our reader may become more familiar with the subject. We will not, however, venture very far among the dry details of crystallography, even if it be a subject of great interest to the student in science. The stone which so readily attracts the eye by its dazzling splendors after it has received a definite form and polish from art, is seldom attractive to the view unless it occurs in a rare and perfect form of crystallization. Even then, in this primitive state it exhibits none of the rainbow play of color which makes the stone so celebrated and so beautiful. In reality, in these rare conditions it is seldom if ever so lustrous and pleasing to the sight as crystals of many other minerals. In general, the diamond is so obscure in its attractions that practised eyes are required for its search.

Recently the distinguished savant Von Tschudi, in visiting some of the diamond mines of Brazil, was unable to distinguish readily the gems as they lay in the washed cascalho, while the trained eyes of the negro miners picked them out with ease. It has been stated that the diamonds are always or nearly always covered with an earthy crust of various hues, especially greenish or yellowish, which is hardly the fact; for what appears to be a crust is often caused by the salient edges of the laminæ, among which a little earthy or coloring matter has been introduced. These extraneous colors generally disappear when the surface of the stone is removed; and, in fact, the degree of their intensity is very much modified when the rough gem is placed in alcohol or in any fluid of high refraction.

The dull, whitish appearance seen in the natural diamonds is also produced by the action of fire, which raises the edges of the laminæ, producing a faint milky aspect.

The primitive form of the mineral is the octahedron, and many irregular masses may be reduced by cleavage to that of a double pyramid. These octahedral crystals are sometimes as perfect in outline as a mathematical model, with clean-cut angles and smooth faces. In some specimens the edges may be truncated, that is, as if they had been flattened or ground off by mechanical means. Generally, however, the crystals are of the form of the octahedrons with rounded faces. The dodecahedrons, with their twelve faces, and the cubes, with their four sides, may also be reduced by cleavage to the primitive form of the double pyramid. Sometimes two crystals are united, forming what are called hemitrope or twin crystals. Then, again, a number may be grouped together, assuming on the whole a globular-like mass. But they are decidedly different from the globular, which in their form of crystallization radiate from the centre of the crystal.

The variety of diamond called boart, or bort, deriving its name from the supposed abortive attempt of nature to form a perfect crystal, is also quite deficient in cleavage, or its laminæ are so irregular as to render splitting quite impossible and the cutting of the stone equally so. The transparency of these forms is also affected by the arrangement of crystallization; hence they are generally crushed into powder for polishing material or used for various purposes in the arts. The specific gravity of these varieties seems to be influenced by the manner of crystallization. For instances, we find that the fine transparent crystals have a specific gravity of 3.55 (water being considered the standard as 1), while the bort is somewhat less; and the massive variety called the carbonado varies from 3 to 3.4, according to the amount of earthy matter it may contain. It has also been asserted that the blue, the green, the orange, and the red varieties are heavier than the white. The phenomena of electricity observed in the diamond are not remarkable, and are inferior to most of the gems. Some of the precious stones when excited retain their electrical properties for hours or even days, but the diamond loses it almost immediately. It exhibits vitreous electricity when rubbed.

Much has been said and written concerning the artificial phosphorescence exhibited by the diamond when removed to a dark room after having been exposed for a short time to the sun’s rays. We are not able to verify this statement, and feel inclined to doubt its correctness, although we have been assured by experimenters of the fact.

One of the most remarkable properties of the diamond is its extreme hardness, in which it far exceeds all known substances in the mineral kingdom. This peculiarity is due to the substance itself, but appears to be modified by its color and its form of crystallization like some other minerals.

The more perfectly the crystal is formed, the easier its laminæ become detached, and the softer the substance appears to be. In the globular forms, which are quite deficient in cleavage planes, the hardness is excessive, and often resists the most determined efforts of the lapidary. Even in fine crystals we shall find that certain angles are harder than others; and we may observe the same relative degree among crystals of other minerals, like those of the topaz. In the large transparent diamonds of irregular form, spots of excessive hardness are often found. These are called by the lapidaries “knots,” and appear to be due to a change in the process of crystallization. The coloring matter, or the mode of its formation, seems to affect the degree of hardness in many minerals; and in respect to the diamond, the rare crystallized black form is harder than the limpid or lighter colored.

Some years ago a black diamond from Borneo was placed in the hand of Gallais the lapidary, to be experimented upon at the expense of the French Institute. The chief object of the test was to ascertain the relative degree of hardness in comparison with some of the other varieties of diamond. In this trial the lapidary wore out his steel wheel and a large quantity of ordinary diamond dust without making the least impression on the surface of the black diamond. Although heavily loaded with weights, it lost none of its roughness, and was heated almost to whiteness by the friction of the wheel, which revolved with great velocity. During the period of this extreme velocity it is reported that a shower of sparks was emitted; but how shall we account for this scintillation, when the ordinary transparent diamond does not give forth sparks when struck by steel?

The carbonado, which is amorphous and without cleavage, is also extremely hard. The term “adamas,” which the ancients bestowed upon it as denoting an invincible infrangibility, is not quite appropriate; for although it is far superior in hardness to all other known substances, it is in reality very fragile. And in the power to resist the effect of shock it is also inferior to some of the other gems, and especially the sapphire. Therefore several mineralogists have thought that the ancients really applied the term to steel or to some of the varieties of corundum, like the ruby and the sapphire, and not the diamond. It is curious that this property should be ascribed erroneously to the diamond for so many ages, when a trifling experiment would have disclosed the real condition of things. In the days of the poet Lucretius the gem was believed to be able to resist violent blows.

——“adamantina saxa
Prima acie constant, ictus contemnere sueta.”

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.”

The color-suite of the mineral is much more extensive and varied than has been generally admitted by mineralogists. We are led to infer from their works that white is the prevailing hue of the gem; but Beudant declares that perfect limpidity and whiteness is rare comparatively, and that the stone is generally affected with yellowish or brownish tints. But what becomes of the vast numbers of these clouded or tinged and inferior gems, if the mines yield so many of them in comparison with the snow-white? Are they consumed in polishing others, or expended in the arts, or have the lapidaries secret processes by which these objectionable tints are expunged from the stone?

Barbot, the French jeweller, declares that, by means of certain particular and energetic agents, aided by a proper degree of heat, he is able to remove the greens of all shades, the light-red, and the yellow, when the coloring matter is superficial, or even situated between the external laminæ. We are inclined to believe Barbot in this particular reference, especially as he admits that he is unable to change much the deep-yellow, the brown, and the smoky-tinted stones. Of the yellow tints, the diamond affords the most beautiful examples, and far surpasses in variety all the other gems, with the exception perhaps of quartz. To the yellow topaz it is decidedly superior in its range of shades, and in some of its chrome-like tints it is without an equal among the gems. This hue of chrome mixed with a faint tinge of green is a delicate, yet gorgeous, shade, and is not often seen. Stones of a canary-yellow are quite common, and perfect resemblances to the Brazilian topaz are not rare. From these hues they pass insensibly into brown and black. The transparent light-brown stones are often modified in hue when exposed to the action of heat, and some of them exhibit remarkable changes of color. M. Halpen, in 1866, exhibited to the French Academy of Sciences a singular diamond of this description. It was a stone of sixty grains weight, and of a whitish hue tinged slightly with brown. But when it was exposed to the action of heat it changed its tint to a fine rose-color, and retained it for six or eight days, when it gradually returned to its natural hue. This remarkable effect was not an accidental result, but was tried five times at the Academy with success and without injury to the stone. In other colored diamonds the action of fire often produces permanent effects, and sometimes a brownish hue is converted into a decided red color. Buckman saw a diamond with a large brown spot in its interior change to a beautiful red, like that of the Balais ruby, after the stone had been placed in borax and exposed to a red heat. Another stone, however, of similar appearance, likewise exposed, changed to a permanent black, to the great injury of the gem and dismay of the experimenter. The red varieties of this mineral are rarely of deep tints, but when they exhibit a decided red color they form the most gorgeous of gems. The largest and finest of this description known is the ten-karat stone purchased by the Emperor Paul of Russia for one hundred thousand roubles. This gem may be considered the marvel of the mineral kingdom. The princely collection of the late Mr. Hope possessed one of a blood-red garnet shade, also a fine twelve and a half karat stone of an apricot hue, besides several others of a beautiful hyacinth red, or of a lilac pink.

The celebrated cabinet of gems belonging to the late Marquis de Drée contained a large and beautiful rose-colored diamond. Prince Riccia, of Naples, acquired in 1830 a very fine rose-colored brilliant of fifteen karats weight. M. Halpen, in 1838, exhibited a magnificent gem of this description of twenty-two and a half karats. Among the crown jewels of France there are several splendid brilliants of a peach-blossom hue, and there are also quite a number to be seen among the princely caskets of Europe. It is, however, somewhat remarkable that this gem, although possessing several shades of red, never, or very rarely, occurs of a decided violet or purple color. Diamonds of a light aqua-marine of greenish and bluish tints are not rare, but those of a positive grass-green color are uncommon. Perfect stones of decided green form the most magnificent gems of this color. The velvety green flashed forth by the extraordinary power of the stone surpasses beyond comparison the finest emeralds with their duller reflections. In fact, we may term the splendid green diamond of forty karats, now in the Green Vaults at Dresden, as being one of the five paragons among all the gems of the world.

In the Museum of Natural History in Paris there are some small diamonds of very fine shades of green, which were collected by the celebrated Werner. Some of the diamonds which have a slight milky hue, when cut so as to allow the play of light within the stone, present a very beautiful appearance. The varied flashes of colored rays, in contrast with the duller hues of the stone, appear like the charming effects of the finest specimens of Siberian adularia, and are therefore entitled to the name of aventurine diamonds.

The asterism, or star-like form of six rays, which is so beautifully displayed by the sapphire when it assumes a certain form of internal arrangement of crystallization, is sometimes, though very rarely, witnessed in the diamond. There is one of this description to be seen in the Museum of the Jardin des Plantes in Paris.

The diamond is rarely found of a perfect shade of blue; but there are now in Europe several magnificent gems of this description. Foremost of all of them stood the famous blue diamond of 67²⁄₁₆ karats, belonging to the French crown. This marvel of Nature’s work, with two other diamonds of paler hue and lesser weight,—thirty-one and ten karats respectively,—disappeared on that fatal night of September, 1792, and have never since been discovered. At the present day, the finest known is that which belongs to the princely collection of the late Mr. Hope, and weighs 44¹⁄₄ karats. It is of a fine blue; but exhibits that steel-like tint which is so often seen in sapphires. The next in value and beauty is that which is preserved at Munich. It is a magnificent gem of thirty-six karats weight, and of superb color.

The crystallized black diamond is a very rare stone; and, when polished, it forms a unique gem, since it exhibits a remarkable brilliancy, proceeding, as it were, from darkness itself. We do not now refer to the compact variety, known as carbon or carbonado, which is never found except in the amorphous form, but the crystalline variety, which is of greater density and more homogeneous. The famous collector, Dogni, possessed a very fine specimen of this kind which had been cut with small facets, and exhibited a vivid eclat. It afterwards came into the possession of Mr. Bapst, who disposed of it to Louis XVIII. for the sum of twenty-four thousand francs. A large and unique diamond, almost black, formerly belonged to the late Duke of York. Several of the European mineralogical cabinets have interesting and valuable collections of colored diamonds; but the finest is to be seen in the Imperial Cabinet of Minerals at Vienna. This beautiful and complete series, which illustrates the great range of the color-suite of the gem, was the life-long labor of a Tyrolese gentleman, by the name of Helmreicher. This enthusiastic amateur went to Brazil, and passed most of his life in the mines, searching for the gems.

We will not fatigue our readers with long quotations of authors and philosophers concerning the spiritual properties of this gem; but we will briefly say, that a well-selected compilation of all these views and speculations, extending back to very early times, would form a chapter by itself, and quite as interesting as absurd. Even the good sense of the Latin philosopher Pliny was affected so far as to indulge in the belief that the gem was not only an antidote to poison, but also freed the mind from vain fears. Late in mediæval times, the adamas was invested with supernatural powers, and regarded as a spiritual creation. And even in the commencement of the seventeenth century Boetius de Boot, in his treatise on gems, asserts that the diamond possesses wonderful metaphysical properties; but remarks that they do not reside in the stone per se, but belong to the angelic spirits whom it has pleased the Almighty to connect, in a mysterious manner, with certain substances in nature.