George Frederick Herbert Smith

CORUNDUM

(Sapphire, Ruby)

RANKING in hardness second to diamond alone, the species known to science as corundum and widely familiar by the names of its varieties, sapphire and ruby, holds a pre-eminent position among coloured gem-stones. The barbaric splendour of ruby (Plate I, Fig. 13) and the glorious hue of sapphire (Plate I, Fig. 11) are unsurpassed, and it is remarkable that the same species should boast such different, but equally magnificent, tints. They, however, by no means exhaust the resources of this variegated species. Fine yellow stones (Plate I, Fig. 12), which compare with topaz in colour and are its superior in hardness, and brilliant colourless stones, which are unfortunately deficient in ‘fire’ and cannot therefore approach diamond, are to be met with, besides others of less attractive hues, purple, and yellowish, bluish, and other shades of green. Want of homogeneity in the coloration of corundum is a frequent phenomenon; thus, the purple stones on close examination are found to be composed of alternate blue and red layers, and stones showing patches of yellow and blue colour are common. Owing to the peculiarity of their interior arrangement certain stones display when cut en cabochon a vivid six-rayed star of light (Plate I, Fig. 15). Sapphire and ruby share with diamond, pearl, and emerald the first rank in jewellery. They are popular stones, especially in rings; their comparative rarity in large sizes, apart from the question of expense, prevents their use in the bigger articles of jewellery. The front of the stones is usually brilliant-cut and the back step-cut, but Indian lapidaries often prefer to cover the stone with a large number of triangular facets, especially if the stone be flawed; star-stones are cut more or less steeply en cabochon.

In composition corundum is alumina, oxide of aluminium, corresponding to the formula Al₂O₃, but it usually contains in addition small quantities, rarely more than 1 per cent., of ferric oxide, chromic oxide, and perhaps other metallic oxides. When pure, it is colourless; the splendid tints which are its glory have their origin in the minute traces of the other oxides present. No doubt chromic oxide is the cause of the ruddy hue of ruby, since it is possible, as explained above (p. 117), closely to imitate the ruby tint by this means, but nothing approaching so large a percentage as 2½ has been detected in a natural stone. The blue colour of sapphire may be due to titanic oxide, and ferric oxide may be responsible for the yellow hue of the ‘oriental topaz,’ as the yellow corundum is termed. Sapphires, when of considerable size, are rarely uniform in tint throughout the stone. Alternations of blue and red zones, giving rise to an apparent purple or violet tint, and the conjunction of patches of blue and yellow are common. Perfectly colourless stones are less common, a slight bluish tinge being usually noticeable, but they are not in much demand because, on account of their lack of ‘fire,’ they are of little interest when cut. The tint of the red stones varies considerably in depth; jewellers term them, when pale, pink sapphires, but, of course, no sharp distinction can be drawn between them and rubies. The most highly prized tint is the so-called pigeon’s blood, a shade of red slightly inclined to purple. The prices for ruby of good colour run from about 25s. a carat for small stones to between £60 and £80 a carat for large stones, and still higher for exceptional rubies. The taste in sapphires has changed of recent times. Formerly the deep blue was most in demand, but now the lighter shade, that resembling the colour of corn-flower, is preferred, because it retains a good colour in artificial light. Large sapphires are more plentiful than large rubies, and prices run lower; even for large perfect stones the rate does not exceed £30 a carat. Large and uniform ‘oriental topazes’ are comparatively common, and realize moderate prices, about 2s. to 30s. a carat according to quality and size. Green sapphires are abundant from Australia, but their tint, a kind of deep sage-green, is not very pleasing. Brown stones with a silkiness of structure are also known.

The name of the species comes through the French corindon from an old Hindu word, korund, of unknown significance, and arose from the circumstance that the stones which first found their way to Europe came from India. At the present day the word corundum is applied in commerce to the opaque stones used for abrasive purposes, to distinguish the purer material from emery, which is corundum mixed with magnetite and other heavy stones of lower hardness. The origin of the word sapphire, which means blue, has been discussed in an earlier chapter (p. 110). Jewellers use it in a general sense for all corundum except ruby. Ruby comes from the Latin ruber, red. The prefix ‘oriental’ (p. 111) is often used to distinguish varieties of corundum, since it is the hardest of ordinary coloured stones and the finest gem-stones in early days reached Europe by way of the East.

Corundum crystallizes either in six-sided prisms terminated by flat faces (Plate I, Fig. 10), which are often triangularly marked, or with twelve inclined faces, six above and six below, meeting in a girdle (Plate I, Fig. 14). Ruby favours the former and the other varieties the latter type. A fine crystal of ruby—the ‘Edwardes,’ so named by the donor, John Ruskin, after Sir Herbert Edwardes—which weighs 33·5 grams (163 carats), is exhibited in the Mineral Gallery of the British Museum (Natural History), and is tilted in such a way that the light from a neighbouring window falls on the large basal face, and reveals the interesting markings that nature has engraved on it. From its type of symmetry corundum is doubly refractive with a direction of single refraction running parallel to the edge of the prism. Owing to the relative purity of the chemical composition the refractive indices are very constant; the ordinary index ranges from 1·766 to 1·774 and the extraordinary index from 1·757 to 1·765, the double refraction remaining always the same, 0·009. The amount of colour-dispersion is small, and therefore colourless corundum displays very little ‘fire.’ The difference between the indices for red and blue light is, however, sufficiently great that the base of a ruby may be left relatively thicker than that of a sapphire to secure an equally satisfactory effect (cf. p. 98)—a point of some importance to the lapidary, since stones are sold by weight and it is his object to keep the weight as great as possible. When a corundum is tested on the refractometer in white light a wide spectrum deliminates the two portions of the field because of the smallness of the colour-dispersion (cf p. 25). The dichroism of both ruby and sapphire is marked, the twin colours given by the former being red and purplish-red, and by the latter blue and yellowish-blue, the second colour in each instance corresponding to the extraordinary ray. Tests with the dichroscope easily separate ruby and sapphire from any other red or blue stone. This character has an important bearing on the proper mode of cutting the stones. The ugly yellowish tint given by the extraordinary ray of sapphire should be avoided by cutting the stone with its table-facet at right angles to the prism edge, which is the direction of single refraction. Whether a ruby should be treated in the same way is a moot point. No doubt if the colour is deep, it is the best plan, because the amount of absorption of light is thereby sensibly reduced, but otherwise the delightful nuances distinguishing ruby are best secured by cutting the table-facet parallel to the direction of single refraction. Yellow corundum also shows distinct dichroism, but by a variation more of the depth than of the tint of the colour; the phenomenon is faint compared with the dichroic effect of a yellow chrysoberyl. The specific gravity also is very constant, varying only from 3·95 to 4·10; sapphire is on the whole lighter than ruby. Corundum has the symbol 9 on Mohs’s scale, but though coming next to diamond it is a very poor second (cf. p. 79). As is usually the case, the application of heat tends to lighten the colour of the stones: those of a pale violet or a yellow colour lose the tint entirely, and the deep violet stones turn a lovely rose colour. On the other hand the action of radium has, as was shown by Bordas, an intensifying action on the colour, and even develops it in a colourless stone. From the latter reaction it may be inferred that often in an apparently colourless stone two or more selective influences are at work which ordinarily neutralize one another, but, being unequally stimulated by the action of radium, they thereupon give rise to colour. The stellate appearance of asterias or star-stones—star-ruby and star-sapphire—results from the regular arrangement either of numerous small channels or of twin-lamellÆ in the stone parallel to the six sides of the prisms; light is reflected from the interior in the form of a six-rayed star (p. 38). Some stones from Siam possess a markedly fibrous or silky structure.

The synthetical manufacture of ruby, sapphire, and other varieties of corundum has already been described (p. 116).

Besides its use in jewellery corundum is on account of its hardness of great service for many other purposes. Small fragments are extensively employed for the bearing parts of the movements of watches, and both the opaque corundum and the impure kind known as emery are in general use for grinding and polishing softer stones, and steel and other metal-work.

The world’s supply of fine rubies is drawn almost entirely from the famous ruby mines near Mogok, situated about 90 miles (145 km.) in a north-easterly direction from Mandalay in Upper Burma and at an elevation of about 4000 ft. (1200 m.) above sea-level. It is from this district that the stones of the coveted carmine-red, the so-called ‘pigeon’s blood,’ colour are obtained. The ruby occurs in a granular limestone or calcite in association with the spinel of nearly the same appearance—the ‘balas-ruby,’ oriental topaz (yellow corundum), tourmaline, and occasionally sapphire. Some stones are found in the limestone on the sides of the hills, but by far the largest quantity occur in the alluvial deposits, both gravel and clay, in the river-beds; the ruby ground is locally known as ‘byon.’ The stones are as a rule quite small, averaging only about four to the carat. Before the British annexation of the country in 1885 the mines were a monopoly of the Burmese sovereigns and were worked solely under royal licence. They are known to be of great antiquity, but otherwise their early history is a mystery. It is said that an astute king secured the priceless territory in 1597 from the neighbouring Chinese Shans in exchange for a small and unimportant town on the Irrawaddy; if that be so, he struck an excellent bargain. The mines were allotted to licensed miners, twin-tsas (eaters of the mine) as they were called in the language of the country, who not only paid for the privilege, but were compelled to hand over to the king all stones above a certain weight. As might be anticipated this injunction caused considerable trouble, and the royal monopolists constantly suspected the miners of evading the regulation by breaking up stones of exceptional size; from subsequent experience, it is probable that large stones were in reality seldom found. Since 1887 the mines have been worked by arrangement with the Government of India by the Ruby Mines, Ltd., an English company. Its career has been far from prosperous, but during recent years, in consequence of the improved methods of working the mines and of the more generous terms afterwards accorded by the Government, greater success has been experienced; the future is, however, to some extent clouded by the advent of the synthetical stone, which has even made its way out to the East.

Large rubies are far from common, and such as were discovered in the old days were jealously hoarded by the Burmese sovereigns. According to Streeter the finest that ever came to Europe were a pair brought over in 1875, at a time when the Burmese king was pressed for money. One, rich in colour, was originally cushion-shaped and weighed 37 carats; the other was a blunt drop in form and weighed 47 carats. Both were cut in London, the former being reduced to 32⁵/₁₆ carats and the latter to 38⁹/₁₆ carats, and were sold for £10,000 and £20,000 respectively. A colossal stone, weighing 400 carats, is reported to have been found in Burma; it was broken into three pieces, of which two were cut and resulted in stones weighing 70 and 45 carats respectively, and the third was sold uncut in Calcutta for 7 lakhs of rupees (£46,667). The finder of another large stone broke it into two parts, which after cutting weighed 98 and 74 carats respectively; he attempted in vain to evade the royal acquisitiveness, by giving up the larger stone to the king and concealing the other. A fine stone, known by the formidable appellation of ‘Gnaga Boh’ (Dragon Lord), weighed 44 carats in the rough and 20 carats after cutting. Since the mines were taken over by the Ruby Mines, Ltd., a few large stones have been discovered. A beautiful ruby was found in the Tagoungnandaing Valley, and weighed 18½ carats in the rough and 11 carats after cutting; perfectly clear and of splendid colour, it was sold for £7000, but is now valued at £10,000. Another, weighing 77 carats in the rough, was found in 1899, and was sold in India in 1904 for 4 lakhs of rupees (£26,667). A stone, weighing 49 carats, was discovered in 1887, and an enormous one, weighing as much as 304 carats, in 1890.

The ruby, as large as a pigeon’s egg, which is amongst the Russian regalia was presented in 1777 to the Czarina Catherine by Gustav III of Sweden when on a visit to St. Petersburg. The large red stone in the English regalia which was supposed to be a ruby is a spinel (cf p. 206).

Comparatively uncommon as sapphires are in the Burma mines a faultless stone, weighing as much as 79½ carats, has been discovered there.

Good rubies, mostly darker in colour than the Burmese stones, are found in considerable quantity near Bangkok in Siam, Chantabun being the centre of the trade, where, just as in Burma, they are intimately associated with the red spinel. Because of the difference in tint and the consequent difference in price, jewellers draw a distinction between Burma and Siam rubies; but that, of course, does not signify any specific difference between them. Siam is, however, most distinguished as the original home of splendid sapphires. The district of Bo Pie Rin in Battambang produces, indeed, more than half the world’s supply of sapphires. In the Hills of Precious Stones, such being the meaning of the native name for the locality, a number of green corundums are found. Siam also produces brown stones characterized by a peculiar silkiness of structure. Rubies are found in Afghanistan at the Amir’s mines near Kabul and also to the north of the lapis lazuli mines in Badakshan.

The conditions in Ceylon are precisely the converse of those obtaining in Burma; sapphire is plentiful and ruby rare in the island. They are found in different rocks, sapphire occurring with garnet in gneiss, and ruby accompanying spinel in limestone, but they come together in the resulting gravels, the principal locality being the gem-district near Ratnapura in the south of the island. The largest uncut ruby discovered in Ceylon weighed 42½ carats; it had, however, a decided tinge of blue in it. Ceylon is also noted for the magnificent yellow corundum, ‘oriental topaz,’ or, as it is locally called, ‘king topaz,’ which it produces.

Beautiful sapphires occur in various parts of India, but particularly in the Zanskar range of the north-western Himalayas in the state of Kashmir, where they are associated with brown tourmaline. Probably most of the large sapphires known have emanated from India. By far the most gigantic ever reported is one, weighing 951 carats, said to have been seen in 1827 in the treasury of the King of Ava. The collection at the Jardin des Plantes contains two splendid rough specimens; one, known as the ‘Rospoli,’ is quite flawless and weighs 132¹⁵/₁₆ carats, and the other is 2 inches in length and 1½ inches in thickness. The Duke of Devonshire possesses a fine cut stone, weighing 100 carats, which is brilliant-cut above and step-cut below the girdle. An image of Buddha, which is cut out of a single sapphire, is exhibited, mounted on a gold pin, in the Mineral Gallery of the British Museum (Natural History).

For some years past a large quantity of sapphires have come into the market from Montana, U.S.A., especially from the gem-district about twelve miles west of Helena. The commonest colour is a bluish green, generally pale, but blue, green, yellow and occasionally red stones are also found; they are characterized by their almost metallic lustre. With them are associated gold, colourless topaz, kyanite, and a beautiful red garnet which is found in grains and usually mistaken for ruby. Rubies are also found in limestone at Cowee Creek, North Carolina.

Blue and red corundum, of rather poor quality, has come from the Sanarka River, near Troitsk, and from Miask, in the Government of Orenburg, Russia, and similar stones have been known at Campolongo, St. Gothard, Switzerland.

The prolific gem-district near Anakie, Queensland, supplies examples of every known variety of corundum except ruby; blue, green, yellow, and parti-coloured stones, and also star-stones, are plentiful. Leaf-green corundum is known father south, in Victoria. The Australian sapphire is too dark to be of much value.

Small rubies and sapphires are found in the gem-gravels near the Somabula Forest, Rhodesia.