CORUNDUM.
The mineral species Corundum affords a number of gems known by different names. These differences arise from the fact that the stones were used as gems before their mineralogical identity was discovered. Thus red Corundum is known as the ruby and blue Corundum as the sapphire. When Corundum suitable for gem purposes occurs of other colors, such as green, yellow or violet, the gems are sometimes known as green, yellow or violet sapphires, respectively, or by the name of another gem which they closely resemble in color, with the adjective Oriental prefixed. Such are the gems known as Oriental topaz, Oriental emerald, Oriental aquamarine, Oriental hyacinth, Oriental amethyst and Oriental chrysolite. Colorless Corundum is known as leucosapphire. While Corundum of all colors is used for gems, it is only that which is transparent which can be so employed. This is sometimes called noble Corundum to distinguish it from common Corundum. The two, however, often occur together. Common Corundum is used as an abrasive, emery being one of its varieties, but it has no gem value.
Corundum is a sesquioxide of aluminum, with the percentages aluminum 53.2, oxygen 46.8. Its hardness is 9 in the scale of which diamond is 10, and no other mineral except the latter equals it in hardness. This hardness gives it a wearing quality as a gem second only to the diamond. The varieties of Corundum differ slightly in hardness, sapphire being the hardest. Noble Corundum has a brilliant, vitreous luster, which, while not equal to that of the diamond, is superior to that exhibited by almost any other gem. Corundum is a heavy mineral, its specific gravity being four times that of water. This high specific gravity affords an easy means of distinguishing the gems of Corundum from those of other species. Corundum is infusible and is not attacked by acids. It crystallizes in the rhombohedral division of the hexagonal system, certain crystal forms being characteristic of the two varieties, ruby and sapphire. Thus ruby tends to crystallize in flat rhombohedral crystals, while sapphire generally forms in longer, hexagonal prisms. (See colored plate in November number.) Corundum is doubly refracting and dichroic. Of the different colors of Corundum above referred to, the blue or sapphire is most common, the red or ruby next. The other colors occur rather sparingly, green having been almost unknown until the discovery of the Montana sapphires. The nature of the coloring ingredient of the different varieties of Corundum is not known, but there is some reason for believing it to be chromium, for Fremy obtained artificial red and blue Corundum by mixing chromium with his other ingredients, after many attempts to obtain the desired color had failed.
Red Corundum varies in hue from rose to deep red. That of the latter tint is the true ruby, the color known as pigeon’s blood being most highly prized. Faultless stones of this color have long been the most valuable of gems, exceeding the diamond in price, weight for weight, unless the latter is colored. Rubies above three carats in weight are about ten times more valuable than ordinary diamonds of the corresponding weights. But few rubies exceeding ten carats are known. The King of Pegu is reported to have one the size of a hen’s egg, but as no one has ever seen it the story may well be doubted. In the crown of the Empress Catherine was, however, one the size of a pigeon’s egg. There is also a large uncut ruby in the British crown, which Ruskin calls the loveliest precious stone of which he has any knowledge.
The chief home of the ruby is Burmah. From its mines and those of Siam and Ceylon have come practically all the world’s supply of rubies. The most important Burmese mines are in Mogouk, ninety miles north of Mandalay. The rubies were evidently formed in limestone, which is now much decomposed, and seem to have been the result of metamorphism of the limestone by the entrance of eruptive rocks. The ruby-bearing earth is known as “byon,” and the stones are obtained from it by washing. The rubies are usually in the form of more or less complete crystals. The mines have been worked since the British occupation of Burmah in 1886 by a British company, and there can be little doubt that a desire to acquire these mines was the chief reason for the occupation. The mines have not proved very profitable, however, and only within the last year or two has the company been able to pay any dividends. The hope of success has lain in the introduction of machinery for washing the byon more cheaply than it could be done by the primitive native methods, and it is now believed by the introduction of an electrical power plant that this has been accomplished. This company now produces at least one-half the annual yield of rubies of the world.
Previous to the English working of the mines the ruby mining was performed by local miners under control of the native government, all rubies above a certain size going to the king. Whenever a ruby of unusual size was found a procession of grandees, with soldiers and elephants, was sent out to meet it. One of the titles of the King of Burmah was Lord of the Rubies.
The Siamese rubies come from near Bangkok, on the Gulf of Siam. They occur in a clay which seems to be the product of alteration of a besalt. These rubies are not equal in quality to those of Burmah. Rubies are also found in the gem gravels of Ceylon and in Afghanistan, thirty-two miles east of Cabul. In our own country ruby Corundum is occasionally found in connection with opaque Corundum in Macon County, North Carolina. In the gravels of Caler Fork of Cowee Creek of this county good rubies are found in sufficient quantity to reward systematic mining for them.
These rubies are mostly small, but some gems of three or four carats’ weight and of excellent color have been obtained.
Among the Montana sapphires an occasional red stone is found, but they do not have the choicest red color.
Another source of rubies is their artificial production, after the method discovered by the French chemist Fremy. These are made by heating a mixture of aluminum sesquioxide, carbonate of lime, barium fluoride and potassium chromate in a porous clay crucible to a temperature of 1500 degrees C. and keeping the mixture fluid for eight days. Well-formed, clear crystals up to one-third of a carat in weight are thus produced, which have the hardness and color of native ruby. They are not considered so valuable as gems as the latter, and can be distinguished by the air bubbles which may be seen with a lens. The expense of making them is nearly equal to the value of native rubies, so that their production is likely to be limited.
Rubies were known to the ancients, being mentioned in the Bible in Proverbs and Job. The Greeks and Romans ascribed to the ruby the power of giving light in the dark, and the Hindoos describe the abodes of their gods as thus lighted. The ruby was much worn as an amulet, being supposed to protect the wearer against plague, poison and evil spirits. It was also thought that it would turn dark if its owner were in danger and would not regain its color until the peril was over.
The ruby is usually cut in the form of the brilliant, like the diamond, but sometimes the step cut is advantageously employed. The stones from India are usually rounded by the native gem cutters and worn in this manner.
Blue precious Corundum or sapphire is more abundant than the red or ruby. Like the red the blue color seems to be due to a content of chromium, since in the artificial crystals already mentioned as produced by Fremy, both colors occur at times in the same crystal. The blue color, however, unlike the reds, disappears on heating.
Blue Corundum exhibits various shades from light to dark, the color most highly prized being that known as cornflower blue. A good sapphire should also have high luster and a velvety sheen. As already noted, sapphire is somewhat harder than ruby, and it is also somewhat heavier. The Montana sapphires are said to be especially hard.
Sapphires have at the present time not over half the value of a ruby of the same size. A price of forty dollars per carat is an average one for a stone of not over ten carats and, as much larger stones are comparatively common, the price does not increase so rapidly as does that of the ruby with an increase in size.
The world’s supply of sapphires comes chiefly from Siam. The most important mines of that country are those of Battambong, a city southeast of Bangkok. The sapphires occur in a sandy clay out of which they are washed. The sapphire-bearing region is about a hundred miles in length. Together with the sapphires occur some rubies, especially in the southern part of the district. Sapphires also occur among the rubies of Burmah, but in small numbers. The so-called gem gravels of Ceylon furnish many sapphires, though their quality is not equal to those of Siam because of paleness of color. In these gem gravels occur also ruby, spinel, garnet, topaz, amethyst, tourmaline and hyacinth. Another locality for sapphires, discovered in the early eighties, is Banskar, in Cashmere, India. These stones were first disclosed by the fall of an avalanche, and later were discovered to exist in the region in considerable numbers. For a time they could be cheaply purchased, but are now jealously guarded by the government. The Montana sapphires have been known since 1865, but were not systematically worked until 1891. They occur in river sands east of Helena, and were first obtained in washing for gold. Now the mother rock has been discovered, and this is mined, the rock being taken out, piled in heaps and submitted to the action of frost through the winter. The sapphires thus become loosened and can be readily separated. These sapphires are well crystallized and are of good average size, though few gems exceed six carats in weight. Their luster and color are for the most part of first quality, and the stones are in demand for the best of jewelry.
Noble Corundum of other colors than those of blue and red is not of abundant occurrence nor is it ordinarily as highly prized as are the sapphire and ruby. Colorless sapphire or leucosapphire is sometimes used as a substitute for the diamond, from which it can readily be distinguished by its lower hardness and higher specific gravity.
Certain specimens of both sapphire and ruby, but especially the former, exhibit when lighted a six-rayed star. This appears as beams of light, radiating from a center, which changes in position as the stone is turned. Such stones are called star or asteriated sapphires or rubies, and are highly prized. They are usually cut with rounded surface, as this best brings out the figure. The cause of the star-shaped figure is generally supposed to be the presence of countless microscopic cavities in the stone, which are arranged parallel to the faces of a six-sided prism. The total reflection of the light from these causes the star. Others think that multitudes of twining lamellæ cause the appearance.
Sapphire is a word which is the same in nearly all languages. In Chaldean, Hebrew, Greek and Latin it has the same form as in modern tongues. This fact testifies to the ancient use of the stone. In early times sapphire was believed to be a destroyer of poison, so that if put into a glass with a spider or venomous reptile it would kill it. It was regarded also as a remedy against fevers.