RUBY AS A GEM
The facets of a cut ruby are ground on a rotating iron disc precisely as in the diamond. The use of diamond-powder as a grinding material is now very general in Europe since it considerably expedites the process. That of polishing follows the operation of grinding, which is effected on a copper disc charged with tripolite moistened with water.
The forms of cutting adopted for the ruby are those generally used for the diamond. The brilliant form is frequently chosen, since this displays the beauties of the stone to the best possible advantage. In order to increase the transparency of the ruby, however, the brilliant is cut thinner and flatter than is allowable in the case of the diamond. Owing to the strong refraction of the ruby, the rays of light which enter the stone by its front facets are totally reflected by the back facets and pass out by the front of the stone, the fine red color of the ruby having been imparted to them during their passage through it. It is this coloring of the rays of light, together with the brilliant luster of the stone, which gives the ruby its effectiveness. Owing to the small dispersion of corundum, the magnificent play of prismatic colors characteristic of the diamond is almost absent in the ruby. This being so, the step-cut or trap-cut form of cutting is just as effective as the brilliant for the ruby, or indeed for any colored stone which shows no play of prismatic colors. The mixed-cut, of which the upper portion consists of brilliant facets and the lower those of the step-cut, is also an effective form. Table-stones, point-stones, and similar forms are scarcely ever cut now; the few examples met with are the work of former times. Flat and thin rubies are usually cut as roses (rosettes), since this form involves little loss of material and, at the same time, produces a good effect. Very small stones are irregularly faceted; they are used to form a contrasting border round some larger precious stone.
In Burma, the chief home of the ruby, the stones are cut en cabochon, that is to say with a rounded surface, before they come on the market. When this form of cutting does not display the beauties of a stone to the best advantage it is recut in Europe. It is obviously to the purchaser's advantage to buy a ruby cut en cabochon rather than an uncut stone, since in the former case it will be possible to detect any faults in the interior. With the exception of the asterias or star-rubies, this gem is seldom in Europe cut en cabochon; in the exceptional case mentioned, the rounded form of cutting is obviously the most suitable for displaying the six-rayed chatoyant star for which the stone is peculiar.
Clear and transparent stones of a full deep color are usually mounted in open settings (a jour); those of poorer quality are often backed by a foil of gold or copper or red glass, which materially improves their appearance. In Burma it is customary, instead of setting such a stone on a foil, to hollow out the underside and fill it in with gold.
Besides being faceted and cut en cabochon, rubies are sometimes engraved with inscriptions or figures, this being most frequently done in the East. Such antique gems of ruby engraved with the head of Jupiter Serapis and a figure of Minerva are known.
While the poorer qualities of ruby are widely distributed, clear, transparent material suitable for cutting is found in but few countries, of which Burma, Siam, and Ceylon are alone of commercial importance at the present time.
Now, just as in former times, Upper Burma furnishes us not only with the finest but also with the largest supply of rubies. The distribution of precious stones (ruby, red tourmaline, jadeite, and amber) in this country is shown on the map above. The ruby mines of Upper Burma were worked at least as early as the fifteenth century and have ever since supplied the greater part of the material used in jewelry, including the finest stones known. The majority of the rubies, which are now put on the market, come from Burma. It is probable; however, that part of this supply is the gradually accumulated stock of former times, and that the yield of the mines is now smaller than formerly.
The Burmese ruby mines were mentioned long ago by Tavernier. According to his account, which, however, was not based on personal observation but en second-hand information, they were situated in the "Capelan Mountains", in Pegu, twelve days journey in a north-east direction from the town of Syriam, now a small village close to Rangoon. The yield at that time (second half of the seventeenth century) was apparently not very great, and was estimated at 100,000 ecus ($2,445,000) per annum by Tavernier, who adds that he found the importation of rubies from Europe into India a lucrative business.
Tavernier's error in describing the locality of the ruby mines has been repeated again and again, and is even now current in the text-books of the present day. There is not the least doubt that the mines referred to are those which are still being worked in Upper Burma, and which are very much further removed from Syriam than Tavernier stated them to be. The distance from here to Mandalay is at least thirty-six days journey, and from Mandalay to the principal ruby district of Mogok is another eight days journey, the less important district of the Sagyin Hills lying, however, a little nearer. For a long time the exact location of these mines was a secret jealously guarded by the Burmese. Since the annexation of the country in 1886 by Britain, more detailed information has been obtainable, and a part of the workings has been taken over by Europeans. The district was officially visited and reported upon in 1888 by Mr. C. Barrington Brown. The rocks and minerals collected there were examined by Professor J. W. Judd, the result of their joint examination being published in 1896 in the Philosophical Transactions of the Royal Society of London.
The district of Mogok is the most important "ruby tract", or "stone tract", and embraces an area of forty-five square miles, or, if some abandoned mines are included, sixty six square miles. The ruby-bearing area is, in all probability, much greater than this, extending to the south and east into the independent Shan States, and has been estimated by Lockhart, who for two years was resident engineer to the Burma Ruby Mining Company, at 400 square miles. This opinion is supported by the recent discovery of an old ruby mine in the river gravels of the Nampai valley, near Namseka village, in the Mainglon State. The district, which is mountainous, and scored by deep valleys, lies to the east of the Irrawaddy, from which it is separated by a plain thirty miles in width, in which a few unimportant ruby mines are worked by the natives.
This district has formed a part of the kingdom of Burma since 1637; its chief town and center of the trade in precious stones is Mogok, latitude 22° 55' N., longitude 96° 30' E. of Greenwich, thirty-four miles in a straight line (but fifty-eight by road) from the river, and ninety miles north-north-east of Mandalay. A little below Mandalay is Ava, formerly known as Ratanapura (city of gems), the old capital of Burma, round which the trade in precious stones of the whole country centers. Mogok stands at an elevation of 4,100 feet above sea level, while the highest point of the district has an elevation of 7,775 feet. In spite of this the country is covered with thick forests, and is unhealthy both for Europeans and natives. The principal mines are situated in the valleys in which stand the towns of Kathay and Kyatpyen (Kapyun). The mountains surrounding the latter town have been conclusively proved by Prinsep to be identical with the "Capelan Mountains" of Tavernier.
The mother-rock of the ruby and of the minerals, such as spinel, with which it is associated, is a white, dolomitic, granular limestone or marble, which forms whole mountain ranges in this district, and which, according to the investigations of Dr. F. Noetling, of the Indian Geological Survey, is of Upper Carboniferous age. These rocks were originally compact limestones of the ordinary kind, which have been altered by contact with intrusive masses of molten igneous rock; this caused the calcium carbonate to re-crystallize out as pure calcite, while the impurities contained in the original limestone crystallized out separately as ruby and its associated minerals. Geologists know the alteration of rocks by contact with a mass of molten igneous material as contact-metamorphism; the results of the process are frequently to be observed in all parts of the world, but, although corundum is often to be found in such altered rocks, fine ruby of gem-quality is only rarely met with. Such were the conclusions as to the geology of the district and the mode of origin of the ruby arrived at by Professor Max Bauer, from information and specimens supplied to him by Dr. F. Noetling, and published in a scientific journal in 1896. The point of view adopted by Mr. C. Barrington Brown and Professor J. W. Judd must not, however, be passed over without notice.
These authors describe the white crystalline limestone, which alone contains the ruby and spinel, as occurring in thick bands interfoliated with gneisses. These gneisses are usually of intermediate chemical composition; but sometimes of more acid, and at other times of more basic character; the crystalline limestones are more intimately associated with the basic gneisses (pyroxene-gneisses and pyroxene-granulites, with pyroxenites and amphibolites). These contain crystals of calcite, and as the proportion of calcite present increases, they merge gradually in the limestones. It is concluded, on these grounds, that the limestones have been derived by the alteration of the lime-feldspar in these basic rocks. This feldspar (anorthite), being a silicate of calcium and aluminum, would, on alteration, give rise to calcium carbonate and hydrated aluminum silicates, the former being deposited as calcite, and the latter as silica (opal), and various aluminum hydroxides (diaspore, gibbsite, bauxite, etc.). Under other conditions of temperature and pressure these may have been afterwards converted into crystallized anhydrous alumina, that is ruby.
In the masses of crystallized limestone occurring in situ precious stones are only sparingly present, being found in much greater abundance in the clayey and sandy weathered products of the mother-rock, which lie on the sides of the hills, fill up the bottom of the valleys, and are often overlain by similar detrital material containing no precious stones. This secondary gem-bearing bed consists of brown or yellow, more or less firm, clayey, and at times sandy, material, known to the Burmese as "byon", which may be regarded as the residue after the solution of the limestones by weathering processes. It contains beside ruby, sapphire, and other color-varieties of corundum, spinel (Tavernier's "mother of ruby"), tourmaline, large fragments of quartz, grains of variously colored feldspars, nodules of weathered iron-pyrites, and other minerals of more or less value, together with fragments of the rocks which occur in situ in the neighborhood. Sometimes in the river alluvium, instead of clayey and sandy material, there are pure gem-sands consisting mainly of minute sparkling grains of ruby.
The gem-bearing layer lies on a soft decomposed rock of characteristic appearance. When the natives reach this level in their excavations they know that the "byon" extends no further down, and that work at that spot must be abandoned. The "byon" lies about 15 to 20 feet below the surface of the floor of the valleys, and is from 4 to 5 feet in thickness, though occasionally it may thin off to a few inches. On the sides of the hills the bed of "byan" may be 15 to 20 feet thick, and sometimes as much as 50 feet.
Diamond Diamond Geology and Mining Diamond Trade
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