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PRECIOUS STONES

RUBY

OCCURRENCE

These garnet-bearing basic rocks include eclogite, amphibolite, and other similar rocks of igneous origin. They have been much weathered, however, and are represented solely by a soft decomposition product known to American petrologists as "saprolite". In this material the rubies occur in "nests" and "bands", and also in what appear to have once been cavities in the original rock. These cavities, when the corundum is pale colored, appear to have been filled with feldspathic material; but when the corundum is of a ruby red color, the surrounding space is filled up with chloritic material. The associated minerals are garnet, in great abundance, sillimanite, kyanite, staurolite (often very clear and gem-like in character), cordierite, zircon, monazite, and others, together with minute quantities of gold.

The corundum found here varies in color from ruby-red through different shades of pink to white. Many of the red crystals exhibit the beautiful so-called pigeon's blood tint, and are in no way inferior to the finest Burmese rubies. Enclosures of various kinds are, however, frequent; these may be extremely minute ("silk" of jewelers), giving rise to cloudiness ("sheen") in the faceted gems, or they may be larger uniform masses of clear red rutile or black ilmenite. Some crystals of ruby have been found to enclose crystals of the variety of garnet known as rhodolite. Enclosures of this kind, however, in no way impair the transparency and beauty of the ruby. Some few specimens of ruby have been found perfectly free from enclosures and large enough to give a cut gem of very fair size. Although the Cowee Creek rubies are very like Burmese stones, yet their mode of occurrence is totally different, for in the former locality the white crystalline limestone of Burma is absent, as are also the fine red spinels so characteristic an associate of the rubies of Burma.

In the State of Montana a few rubies have been found in association with sapphire; they are usually of a pale rose-red color like those of Ceylon, stones of a fine deep color being only occasionally met with. It is hoped that a more systematic working of these deposits, especially at Ruby Bar, will result in more frequent discoveries of stones of good color. In America, as in Australia, garnets have been frequently mistaken for the more costly ruby, and have been collected and sold as such. In Europe red corundum, suitable for cutting as gems, is practically absent, and the same is the case in the continent of Africa, the so-called "Cape rubies", occurring in South Africa in association with diamond, being not ruby but garnet.

ARTIFICIAL PRODUCTION

Ruby is the only valuable precious stone, which hitherto has been produced by artificial means in crystals of fair size showing all the characters of the natural minerals. The honor of this achievement belongs to the French chemist Fremy, whose efforts in this direction have, after many trials, at last been crowned with success. His object was attained by fusing together in an earthen crucible at a high temperature (1500° C.), a mixture of perfectly pure alumina (Al2O3), potassium carbonate, barium (or calcium) fluoride, and a small amount of potassium chromate, the whole mass being kept in a molten state for a week. The series of reactions, which take place under these conditions probably, begins with the formation of aluminum fluoride. This compound, as a result of contact with the moisture of the atmosphere and furnace gases-a contact rendered possible by the porous nature of the crucible-yields aluminum oxide (alumina). This, by taking up chromic oxide from the potassium chromate, assumes a red color and crystallizes out as ruby. When isolated, after cooling, from the fused mass, in which the crystals are embedded, they are found to differ in nowise from naturally occurring crystals of ruby.

The artificial crystals so formed have always the form of a rhombohedron in combination with extensively developed basal planes, faces which bound natural crystals of ruby also, as shown in Fig. a-d. The thin tabular crystals produced by this method are always of small size, never exceeding 1/3 carat in weight. Their size is increased when larger amounts of material are allowed to interact in the crucible. The color of the artificial product varies from pale to dark red, according to the conditions of the experiment. The most beautiful and characteristic ruby-red color was produced by the addition of 3 to 4 per cent of potassium chromate. The resulting color seem, however, somewhat difficult to control, for the crystals often more or less incline to a violet color, sometimes, indeed, being quite blue, while crystals colored red at one end and blue at the other have been occasionally produced. From these observations Fremy concludes that the color of naturally occurring sapphires, as well as of rubies, may be due to chromium. More than 3 to 4 per cent of the chromium salt is taken up only with difficulty, and the crystals receive a violet tint, differing very markedly from the color of naturally occurring rubies. Artificially formed rubies which, of course, differ in no way from naturally occurring stones, except in their mode of formation, have been mounted as gems, both in a cut and in an uncut condition. Having the same hardness as natural corundum, they have also been utilized as the pivot-supports of watches.

The cost of production of artificially made rubies is so high that they are no cheaper than stones formed by nature; moreover, their small size strictly limits their general application. Before his death Fremy expressed a hope that crystals of much greater size would result from experiments conducted in a crucible of 50 liters capacity. However this may be, there is no immediate prospect of the artificial ousting the natural product. Other investigators, experimenting in the same direction, have been successful in producing crystals of corundum, notably J. Morozewicz. His fused silicates yielded crystals of spinel as well as of corundum. The corundum crystals were tabular in habit and reached a diameter of 1.5 millimeters. The various colors-red, blue, yellow, and greenish-yellow of these crystals must have been due to the presence of iron, for in the experiments of this investigator chromium was not an ingredient of the fused mass.

The peculiar, fine carmine-red rubies of considerable size and unknown origin, which appeared in the market in 1885 at Geneva, may be mentioned here. They have the hardness and specific gravity of the natural mineral, but are less brilliant, and in all probability are artificial products. The color as seen in the spectroscope is more like that of the artificial crystals prepared by Fremy than of natural crystals, and, moreover, certain appearances under the microscope point in the same direction. The origin of these stones is mysterious, and, if artificial, nothing as to their mode of preparation is known. According to one report they have been formed by fusing together several small rubies; this, however, is scarcely credible, since at the extremely high temperature of the melting-point of corundum the ruby assumes a dull gray color. Some authorities again have supposed them to he formed by a method analogous to Fremy's, while yet others have supposed each stone to consist of several small rubies held together in a matrix of glass of the same color and refractive index. The success, which has been attained so far in the artificial production of rubies is encouraging, and affords grounds for the hope that still greater achievements will be possible to future investigators.

DISTINCTION FROM OTHER RED STONES

It is only natural that attempts should be made to substitute for the costly ruby some less valuable stone of similar color. The two stones most frequently passed off' as, and mistaken for, rubies are spinel and garnet. The so-called rubies of cheap jewelry are in reality either the variety of spinel, known as "ruby-spinel", or red tourmaline (rubellite), while topaz may be substituted for pale red ruby. Red quartz will scarcely pass as a substitute for ruby, but red glass (paste) is frequently so passed off.

The crystalline form of the above-mentioned substitutes will, if exhibited, serve to distinguish each from the ruby. In either the rough or cut condition, spinel, garnet, and glass may be readily distinguished from ruby by their single refraction and the absence in them of dichroïsm. Red tourmaline and quartz have a much lower specific gravity than ruby; the latter sinks heavily in methylene iodide, while the two former floats easily. Rose-red topaz can only be substituted for pale rose-red ruby; since there is little difference between them in value it is not so important from a pecuniary point of view to be able to distinguish the one from the other. The specific gravity, however, affords a distinguishing characteristic, since topaz (sp. gr. = 3.5 g/cm3) floats in the heaviest liquid, while ruby (sp. gr. = 4.0 g/cm3) sinks. One can scarcely fail to distinguish the ruby, from any stone, which may be substituted for it, by its great hardness. After the diamond corundum is the hardest of all known minerals, and will scratch any of the stones mentioned above with ease. The word ruby is often used in the designation of stones belonging to mineral species other than corundum. Rose-quartz, for instance, is known as "Bohemian ruby", rose-red topaz as "Brazilian ruby", red garnet as "Cape ruby", and also as "Adelaide ruby"; "Siberian ruby" is the name given to red tourmaline (rubellite), and "false ruby" to fluorspar; while certain varieties of spinel are referred to as "ruby-spinel" and" balas-ruby".

In the manufacture of the so-called ruby-glass various pigments have been used for the purpose of reproducing the color of the ruby. Manganese salts give a fairly close imitation, but the color, which results from their use, is too strongly violet. The best results are obtained with gold salts, purple of cassius, etc., which are fused with the glass or strass. The use of gold salts necessitates the greatest care; otherwise the glass will be cloudy. A glass colored with gold salts after first being cooled is yellowish-green, the fine red color only appearing after the glass has been annealed, an operation, which is known as "tinting". By the use of gold salts, glass of the finest ruby-red color can be obtained, and by varying the percentage of gold in the strass different shades of color are produced. It is an interesting fact that fine ruby-glass has been found in ancient Celtic graves.

[ RUBY  1  2  3  4  5  SAPPHIRE  7  8  9  EMERALD  11  12  13  AQUAMARINE  15  ]
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This document is in the public domain.

March, 2011