In chemistry, corundum is pure alumina, the oxide of the metal aluminum, composed of 53.2 per cent. of the metal and 46.8 per cent. of oxygen. Natural corundum is probably never chemically pure; the inclusions of foreign elements, sometimes but the merest traces, impart the colour that makes the gem. When foreign matter is present in large proportion corundum is impossible for gem purposes, although of great value industrially; inferior translucent specimens serve for pivot supports of watches and other delicate machines and the opaque as an abrasive; thus common corundum is used for cutting and polishing gem minerals lower in the scale of hardness than the diamond, a variety of it being the common compact black emery powder used for sharpening and polishing in mechanical and domestic uses, and familiar to everyone.
A chemical analysis of a fine specimen of an "Oriental ruby," of the approved rich deep red hue was as follows: alumina, 97.32; iron oxide, 1.09; silica, 1.21; in all, 99.62. The extent to which crystallography goes and its fine, yet plain, distinctions, in determining gem minerals, are illustrated by the marked crystallographic differences between the ruby and the sapphire, which differ but slightly in chemical composition, having the same constituents but different proportions; thus one typical sapphire analysed entire exhibited alumina, 97.51; iron oxide, 1.89; and silica, 0.80; in all, 100.20. The forms of corundum generally occur in two different habits represented by the ruby and the sapphire; in the former the prism predominates and in the latter the hexagonal pyramid.
Although corundum is second to the diamond in point of hardness, it is approached much more closely by the minerals next below it in the scale of hardness, than it approaches the eminent and reserved diamond.
Pure corundum has a high specific gravity ranging from 3.94 to 4.08, and this great density makes the specific gravity test in distinguishing it from other stones both easy and important. The differently coloured varieties have not been proved to vary in this particular. Acids will not attack corundum nor is it fusible before the blowpipe. Some specimens when heated in the dark are beautifully phosphorescent. Corundum, by friction, develops positive electricity, which it retains for some time. The lustre of corundum and its fire approach these qualities in the diamond, but the lustre is vitreous instead of adamantine, although it is very durable. Corundum is optically uniaxial and strongly doubly refracting, but the dispersion produced is slight and it is, therefore, incapable of emitting flashes of prismatic colours like the diamond. Coloured corundum crystals are dichroic and the deeper the colour the more pronounced the dichroism. A constant characteristic of coloured corundum gems is that they are as beautiful by artificial light as by daylight.
There are at least nine varieties of corundum used as gems and familiar to nearly all jewellers; the coloured varieties, other than the red ruby and blue sapphire, are named for the gems of other mineral species that they resemble in colour, only with the distinguishing prefix of "Oriental." The arbitrary names and colours are: Ruby ("Oriental ruby"), red; Sapphire ("Oriental sapphire"), blue; Leuco-sapphire (White sapphire), colourless; "Oriental aquamarine," light bluish-green; "Oriental emerald," green; "Oriental chrysolite," yellowish-green; "Oriental topaz," yellow; "Oriental hyacinth," aurora red; "Oriental amethyst," violet.
The colour-varieties of corundum are found in irregular grains and as crystals embedded in some old crystalline rock, as granite or gneiss. The gem-varieties frequently occur as secondary contact minerals, which contact with a molten igneous rock has developed in limestone. These embedded crystals are frequently liberated by the weathering and uncovering of such rocks, and then the crystals are found in the debris in the beds of streams.
Red corundum is supposed to be identical with the anthrax mentioned by Theophrastus and to have been termed carbuncle during the Middle Ages. The colour-tone of the ruby varies greatly, and the presence of deep, intense tones of red causes the term "masculine" to be applied to a gem, while the paler tints suggest the term "feminine." Rubies range from a delicate pink tint through pale rose red to reddish-white, pure red, carmine red, or blood red. A tinge of blue or violet is frequently discernible in these shades. The desired tone in ruby colour was so aptly compared by the Burmese to the blood of a freshly-killed pigeon that the term "pigeon-blood" is the accepted qualification for the colour of the choicest and costliest ruby gems. The colouring is not always uniform, there sometimes occurring alternate layers of colours and colourless stone; a process of heating usually renders the colour uniform. The ruby does not lose its colour when heated, and hence it is assumed that the colouring matter is not organic, as in that case it would be destroyed, but is probably due to a trace of chromium. A graduated increase of heat will not fracture the stone, which upon cooling becomes white, then green, and finally regains its original red colour. The ruby is dichroic according to the direction in which it is seen, and in cutting it this must be taken into consideration; the table-the largest facet surface-should be aligned with the basal planes of the crystal, in order to exhibit the greatest possible depth of colour. The dichroism of the ruby is one of its certain distinctions from spinel, garnet, and other red stones which crystallise in the cubic system and therefore are but singly refracting.
Rubies sometimes show on their basal planes, or on a convex surface which corresponds to the bases, a six-rayed star of gleaming light; these are called asteriated rubies, "star-rubies," or ruby cat's-eye.
So valuable are flawless rubies of good colour, that when they ascend much above a carat in weight their prices depend to a considerable extent on fancy. A three-carat ruby of desirable qualities is a rarity, while three-carat diamonds are common. Although nothing will definitely indicate what a fine ruby of three carats and upward might bring in the open market, yet Dr. George F. Kunz appraised a fine ruby of 9 5-16 carats at $33,000, and Mr. E. W. Streeter, the London jeweller and author, records a purchase price of about $50,000 for a cut ruby of 32 5/16 carats.
The common faults of rubies are lack of clearness; the presence of "clouds," also termed silk, especially in light-coloured stones; patches which resemble milk ("chalcedony patches"); internal cracks and fissures ("feathers"); and the colour being unequally distributed.
From the beginning of its history the main supply of the beautiful ruby gem has been from a small territory in upper Burma, whence, also, have come those of the finest quality. The centre of this mining region and the ruby trade is the town of Mogok, ninety miles north-north-east of Mandalay. The mining district ranges from four thousand to nearly eight thousand feet above sea-level, but, despite its altitude, this forest-covered region proves unhealthy for Europeans. The principal mines are in two valleys in which are the towns of Kathay and Kyatpyen.
Rubies and the minerals with which they are associated, such as spinel, are here found in a mother-rock of white, dolomitic, granular limestone or marble, of the upper Carboniferous age. These rocks have been altered by contact with molten igneous material which recrystallised the calcium carbonate as pure calcite, while the impurities became the ruby and its associated minerals. The precious stones are but occasionally found in the rock itself, but in an adjacent ground, which the miners call "byon," where the gem stones have weathered out; in the neighbouring river alluvium are found ruby particles, called ruby-sand. Prior to 1886 the rubies were mined by the Burmese with the primitive methods that had been in vogue for centuries, but when, in that year, Burma became part of the British Empire, the work was taken up first by an Anglo-Italian and then by an English company, which paid the Indian Government for this concession of mining rights the equivalent of about $125,000 annually.
Siam has long produced corundum rubies, but the gems are usually darker and inferior to the beautiful clear red stones from Burma. The principal mines are controlled by an English company. A few rubies have come from the gem-sands of Ceylon; a few have been found in Mysore and Madras, India; and inconsiderable products in Afghanistan and Australia. Rubies have been found in North Carolina and Montana in the United States, but the products are not of commercial importance.
Corundum rubies formed of ruby material by artificial methods have attracted attention and are cutting some figure in the jewelry trade, but they are not and can never be the peers of natural rubies; man's ingenuity and science cannot compete with Nature in the gem business. Artificial rubies are described in another chapter.
Of the other stones than corundum called "ruby," the only important ones are the varieties of spinel, which chemically is closely allied to corundum so that the red varieties of spinel might be regarded as cousins-german to the real ruby. The "Cape ruby"-so called in the jewelry trade-is pyrope garnet from the diamond-bearing rock of South Africa, and is described in its proper place-the chapter on the garnet. Stones sometimes substituted for the ruby by dealers, or mistakenly called rubies, are red tourmaline, or rubellite, called "Siberian ruby"; rose topaz, called "Brazilian ruby"; and hyacinth or jacinth, which is zircon, and is described in the chapter on "Semi-Precious Stones Occasionally Used." Spinel has perhaps a wider range of colour than almost any other mineral, but it will be considered here chiefly with regard to the red varieties approximating the colour of the ruby. Spinel is practically a magnesium aluminate, consisting of alumina, 71.8%, and magnesia, 28.2%. The chief red shades are: deep red, Siam ruby and spinel ruby; rose red, balas ruby; yellow or orange red, rubicelle; violet red, almandine ruby. The native name in India for spinel is "pomegranate." A slight knowledge of mineralogy should suffice to distinguish the corundum ruby from its spinel distant relative, for the latter is less hard and of lower specific gravity, and different in crystallisation. Spinel is of about the hardness of topaz, or 8 in the Mohs scale, and its specific gravity is about 3.6. It crystallises in the isometric system and usually appears in the form of octahedrons. It is singly refracting, corundum doubly. Spinel is infusible before the blowpipe, but heating it will cause it to undergo several changes of colour, ultimately returning to its original hue, so that it might be termed the chameleon of gem minerals. Without any design to substitute spinel for corundum rubies, spinel has its own deserved value, and its beauty and intrinsic worth deserve for it an inclusion in the company of the high-class gems. It is interesting to note that spinel ruby is not only the relative of the patrician corundum ruby, but the poor relation dwells together with its wealthy relative in nature. Both rubies are found associated in the gem gravels of Ceylon, Siam, Australia, and Brazil, as well as in the crystalline limestone of upper Burma. Spinel rubies are found in quantity in Balakschan, Afghanistan, near the River Oxus; the name "Balas ruby" is probably derived from Beloochistan, otherwise Balakschan.
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Precious Stones Guide Vol 4
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