Gem-Stones and Their Distinctive Characters

CHAPTER XIV

MANUFACTURED STONES

THE initial step in the examination of a crystallized substance is to determine its physical characters and to resolve it by chemical analysis into its component elements; the final, and by far the hardest, step is to build it up or synthetically prepare it from its constituents. Unknown to the world at large, work of the latter kind has long been going on within the walls of laboratories, and as the advance in knowledge placed in the hands of experimenters weapons more and more comparable with those wielded by nature, their efforts have been increasingly successful. So stupendous, however, are the powers of nature that the possibility of reproducing, by human agency, the treasured stones which are extracted from the earth in various parts of the globe at the cost of infinite toil and labour has always been derided by those ignorant of what had already been accomplished. Great, therefore, was the consternation and the turmoil when concrete evidence that could not be gainsaid showed that man’s restless efforts to bridle nature to his will were not in vain, and congresses of all the high-priests of jewellery were hastily convened to ban such unrighteous products, with what ultimate success remains to be seen.

Crystallization may be caused in four different ways, of which the second alone has as yet yielded stones large enough to be cut—

1. By the separation of the substance from a saturated solution. In nature the solvent may not be merely hot water, or water charged with an acid, but molten rock, and the temperature and the pressure may be excessively high.

2. By the solidification of the liquefied substance upon cooling. Ice is a familiar example of this type.

3. By the sublimation of the vapour of the substance, which means the direct passage from the vapour to the solid state without traversing the usually intervening liquid state. It is usually the most difficult of attainment of the four methods; the most familiar instance is snow.

4. By the precipitation of the substance from a solution when set free by chemical action.

Other things being equal, the simpler the composition the greater is the ease with which a substance may be expected to be formed; for, instead of one complex substance, two or more different substances may evolve, unless the conditions are nicely arranged. Attempts, for instance, to produce beryl might result instead in a mixture of chrysoberyl, phenakite, and quartz.

By far the simplest in composition of all the precious stones is diamond, which is pure crystallized carbon; but its manufacture is attended by well-nigh insuperable difficulties. If carbon be heated in air, it burns at a temperature well below its melting point; moreover, unless an enormously high pressure is simultaneously applied, the product is the other form of crystallized carbon, namely, the comparatively worthless graphite. Moissan’s interesting course of experiments were in some degree successful, but the tiny diamonds were worthless as jewels, and the expense involved in their manufacture was out of all proportion to any possible commercial value they might have.

Next to diamond the simplest substances among precious stones are quartz (crystallized silica) and corundum (crystallized alumina). The crystallization of silica has been effected in several ways, but the value in jewellery of quartz, even of the violet variety, amethyst, is not such as to warrant its manufacture on a commercial scale. Corundum, on the other hand, is held in high esteem; rubies and sapphires, of good colour and free from flaws, have always commanded good prices. The question of their production by artificial means has therefore more than academic interest.

Ever since the year 1837, when Gaudin produced a few tiny flakes, French experimenters have steadily prosecuted their researches in the crystallization of corundum. FrÉmy and Feil, in 1877, were the first to meet with much success. A portion of one of their crucibles lined with glistening ruby flakes is exhibited in the British Museum (Natural History).

Fig. 53.—Verneuil’s
Inverted Blowpipe.

In 1885 the jewellery market was completely taken by surprise by the appearance of red stones, emanating, so it is alleged, from Geneva; having the physical characters of genuine rubies, they were accepted as, and commanded the prices of, the natural stones. It was eventually discovered that they had resulted from the fusion of a number of fragments of natural rubies in the oxy-hydrogen flame. The original colour was driven off at that high temperature, but was revived by the previous addition of a little bichromate of potassium. Owing to the inequalities of growth, the cracks due to rapid cooling, the inclusion of air-bubbles, often so numerous as to cause a cloudy appearance, and, above all, the unnatural colour, these reconstructed stones, as they are termed, were far from satisfactory, but yet they marked such an advance on anything that had been accomplished before that for some time no suspicion was aroused as to their being other than natural stones.

A notable advance in the synthesis of corundum, particularly of ruby, was made in 1904, when Verneuil, who had served his apprenticeship to science under the guidance of FrÉmy, invented his ingenious inverted form of blowpipe (Fig. 53), which enabled him to overcome the difficulties that had baffled earlier investigators, and to manufacture rubies vying in appearance after cutting with the best of nature’s productions. The blowpipe consisted of two tubes, of which the upper, E, wide above, was constricted below, and passing down the centre of the lower, F, terminated just above the orifice of the latter in a fine nozzle. Oxygen was admitted at C through the plate covering the upper end of the tube, E. A rod, which passed through a rubber collar in the same plate, supported inside the tube, E, a vessel, D, and at the upper end terminated in a small plate, on which was fixed a disc, B. The hammer, A, when lifted by the action of an electromagnet and released, fell by gravity and struck the disc. The latter could be turned about a horizontal axis placed eccentrically, so that the height through which the hammer fell and the consequent force of the blow could be regulated. The rubber collar, which was perfectly gas-tight, held the rod securely, but allowed the shocks to be transmitted to the vessel, D, an arrangement of guides maintaining the slight motion of the vessel strictly vertical. This vessel, which carried the alumina powder used in the manufacture of the stone, had as its base a cylindrical sieve of fine mesh. The succession of rapid taps of the hammer caused a regular feed of powder down the tube, the amount being regulated by varying the height through which the hammer fell. Hydrogen or coal-gas was admitted at G into the outer tube, F, and in the usual way met the oxygen just above the orifice, L. To exclude irregular draughts, the flame was surrounded by a screen, M, which was provided with a mica window, and a water-jacket, K, protected the upper part of the apparatus from excessive heating.

Fig. 54.—‘Boule,’
or Pear-shaped
Drop.

The alumina was precipitated from a solution of pure ammonia—alum, (NH₄)₂SO₄.Al₂(SO₄)₃.24H₂O, in distilled water by the addition of pure ammonia, sufficient chrome-alum also being dissolved with the ammonia-alum to furnish about 2½ per cent. of chromic oxide in the resulting stone. The powder, carefully prepared and purified, was placed, as has been stated above, in the vessel, D, and on reaching the flame at the orifice it melted, and fell as a liquid drop, N, upon the pedestal, P, which was formed of previously fused alumina. This pedestal was attached by a platinum sleeve to an iron rod, Q, which was provided with the necessary screw adjustments, R and S, for centring and lowering it as the drop grew in size. Great care was exercised to free the powder from any trace of potassium, which, if present, imparted a brownish tinge to the stone. The pressure of the oxygen, low initially both to prevent the pedestal from melting, and to keep the area of the drop in contact with the pedestal as small as possible, because otherwise flaws tended to start on cooling, was gradually increased until the flame reached the critical temperature which kept the top of the drop melted, but not boiling. The supply of powder was at the same time carefully proportioned to the pressure. The pedestal, P, was from time to time lowered, and the drop grew in the shape of a pear (Fig. 54), the apex of which was downwards and adhered to the pedestal by a narrow stalk. As soon as the drop reached the maximum size possible with the size of the flame, the gases were sharply and simultaneously cut off. After ten minutes or so the drop was lowered from the chamber, M, by the screw, S, and when quite cold was removed from the pedestal.

Very few changes have been made in the method when adapted to commercial use. Coal-gas has, however, entirely replaced the costly hydrogen, and the hammer is operated by a cam instead of an electromagnet, while, as may be seen from the view of a gem-stone factory (Plate XIV), a number of blowpipes are placed in line so that their cams are worked by the same shaft, a. The fire-clay screen, b, surrounding the flame is for convenience of removal divided into halves longitudinally, and a small hole is left in front for viewing the stone during growth, a red glass screen, c, being provided in front to protect the eyes from the intense glare. Half the fire-clay screen of the blowpipe in the centre of the Plate has been removed to show the arrangement of the interior. The centring and the raising and lowering apparatus, d, have been modified. The process is so simple that one man can attend to a dozen or so of these machines, and it takes only one hour to grow a drop large enough to be cut into a ten-carat stone.

PLATE XIV

BLOWPIPE USED FOR THE MANUFACTURE OF RUBIES AND SAPPHIRES

The drops, unless the finished stone is required to have a similar pear shape, are divided longitudinally through the central core into halves, which in both shape and orientation are admirably suited to the purposes of cutting; as a general rule, the drop splits during cooling into the desired direction of its own accord.

Fig. 55.—Bubbles
and Curved StriÆ in
Manufactured Ruby.

Each drop is a single crystalline individual, and not, as might have been anticipated, an alumina glass or an irregular aggregation of crystalline fragments, and, if the drop has cooled properly, the crystallographic axis is parallel to the core of the pear. The cut stone will therefore have not only the density and hardness, but also all the optical characters—refractivity, double refraction, dichroism, etc.—pertaining to the natural species, and will obey precisely the same tests with the refractometer and the dichroscope. Were it not for certain imperfections it would be impossible to distinguish between the stones formed in Nature’s vast workshop and those produced within the confines of a laboratory. The artificial stones, however, are rarely, if ever, free from minute air-bubbles (Fig. 55), which can easily be seen with an ordinary lens. Their spherical shape differentiates them from the plane-sided cavities not infrequently visible in a natural stone (Fig. 56). Moreover, the colouring matter varies slightly, but imperceptibly, in successive shells, and consequently in the finished stone a careful eye can discern the curved striations (Fig. 55) corresponding in shape to the original shell. In a natural stone, on the other hand, although zones of different colours or varying shades are not uncommon, the resulting striations are straight (Fig. 56), corresponding to the plane faces of the original crystal form. By sacrificing material it might be possible to cut a small stone free from bubbles, but the curved striations would always be present to betray its origin.

Fig. 56.—Markings in Natural Ruby.

The success that attended the manufacture of ruby encouraged efforts to impart other tints to crystallized alumina. By reducing the percentage amount of chromic oxide, pink stones were turned out, in colour not unlike those Brazilian topazes, the original hue of which has been altered by the application of heat. These artificial stones have therefore been called ‘scientific topaz’; of course, quite wrongly, since topaz, which is properly a fluo-silicate of aluminium, is quite a different substance.

Early attempts made to obtain the exquisite blue tint of the true sapphire were frustrated by an unexpected difficulty. The colouring matter, cobalt oxide, was not diffused evenly through the drop, but was huddled together in splotches, and it was found necessary to add a considerable amount of magnesia as a flux before a uniform distribution of colour could be secured. It was then discovered that, despite the colour, the stones had the physical characters, not of sapphire, but of the species closely allied to it, namely, spinel, aluminate of magnesium. By an unsurpassable effort of nomenclature these blue stones were given the extraordinary name of ‘Hope sapphire,’ from fanciful analogy with the famous blue diamond which was once the pride of the Hope collection. A blue spinel is occasionally found in nature, but the actual tint is somewhat different. These manufactured stones have the disadvantage of turning purple in artificial light. By substituting lime for magnesia as a flux, Paris, a pupil of Verneuil’s, produced blue stones which were not affected to the same extent. The difficulty was at length overcome at the close of 1909, when Verneuil, by employing as tinctorial agents 0·5 per cent. of titanium oxide and 1·5 per cent. of magnetic iron oxide, succeeded in producing blue corundum; it, however, had not quite the tint of sapphire. Stones subsequently manufactured, which were better in colour, contained about 0·12 per cent. of titanium oxide, but no iron at all.

By the addition to the alumina of a little nickel oxide and vanadium oxide respectively, yellow and yellowish green corundums have been obtained. The latter have in artificial light a distinctly reddish hue, and have therefore been termed ‘scientific alexandrite’; of course, quite incorrectly, since the true alexandrite is a variety of chrysoberyl, aluminate of beryllium, a very different substance.

If no colouring matter at all be added and the alum be free from potash, colourless stones or white sapphires are formed, which pass under the name ‘scientific brilliant.’ It is scarcely necessary to remark that they are quite distinct from the true brilliant, diamond.

The high prices commanded by emeralds, and the comparative success that attended the reconstruction of ruby from fragments of natural stones, suggested that equal success might follow from a similar process with powdered beryl, chromic oxide being used as the colouring agent. The resulting stones are, indeed, a fair imitation, being even provided with flaws, but they are a beryl glass with lower specific gravity and refractivity than the true beryl, and are wrongly termed ‘scientific emerald.’ Moreover, recently most of the stones so named on the market are merely green paste.

It is unfortunate that the real success which has been achieved in the manufacture of ruby and sapphire should be obscured by the ill-founded claims tacitly asserted in other cases.

At the time the manufactured ruby was a novelty it fetched as much as £6 a carat, but as soon as it was discovered that it could easily be differentiated from the natural stone, a collapse took place, and the price fell abruptly to 30s., and eventually to 5s. and even 1s. a carat. The sapphires run slightly higher, from 2s. to 7s. a carat. The prices of the natural stones, which at first had fallen, have now risen to almost their former level. The extreme disparity at present obtaining between the prices of the artificial and the natural ruby renders the fraudulent substitution of the one for the other a great temptation, and it behoves purchasers to beware where and from whom they buy, and to be suspicious of apparently remarkable bargains, especially at places like Colombo and Singapore where tourists abound. It is no secret that some thousands of carats of manufactured rubies are shipped annually to the East. Caveat emptor.

Clyx.com