Chapter 9
Section 9 of Birds and Nature, Volume 12, Number 5, December 1902. For more information or to volunteer, please visit LibriVox.org. Corundum and Spinelle. 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 color 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 Fremmy 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 Burma. 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 Moguk, 90 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 biome, 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 Burma 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 buy-on 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 Burma was the lord of 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 basalt.
These rubies are not equal in quality to those of Burma. Rubies are also found in the gem gravels of Ceylon and in Afghanistan, 32 miles east of Kabul. In our own country, ruby corundum is occasionally found in connection with opaque corundum in Macon County, North Carolina. In the gravels of Kaler Fork of Coey 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 redstone 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 Frémy. 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 centigrade, 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 Hindus described 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 Fremmy, 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. 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 $40 per carat is an average one for a stone of not over 10 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 Battambang, 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 Burma, 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 80s is Bangskar in Kashmir, 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 leukosapphire is sometimes used as a substitute for the diamond,
from which it can be readily 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 lamellae 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. Spinelle. The group of spinel includes, in mineralogy, a number of species of different, though analogous, composition.
The spinel employed as a gem is almost wholly confined to the magnesium aluminate, having the percentage composition alumina 71.8 and magnesia 28.2. This is usually of a red color, different shades giving gems known by different names as follows. Deep red, spinel ruby, rose red, ballast ruby, yellow or orange red, rubicel, violet red, almendene ruby. Spinel is thus known among gems chiefly as a relative of the ruby,
and this sort of spinel will first be considered. The spinel rubies differ from the true or corundum rubies in hardness, specific gravity, and system of crystallization. The hardness of spinel is 8, or about that of topaz, and the specific gravity 3.6. It is thus neither as hard nor as heavy as corundum ruby.
Again, the system of crystallization differs. Spinel crystallizes in the isometric system and is usually found in the form of octahedrons, while corundum ruby is hexagonal in crystallization. See colored plate in November number. Spinel is singly refracting in polarized light
and corundum doubly refracting. Spinel ruby is infusible before the blowpipe, but on heating undergoes a curious series of changes in color which are quite characteristic. The red changes first to brown and then becomes black and opaque,
but on cooling, the black changes to green, then becomes nearly colorless, and finally, the stone resumes its original red color. As a small percentage of chromium is usually found by analysis to exist in ruby spinel, its color is generally considered to be due to this ingredient. While the spinel ruby is considered of less value than the corundum ruby,
and is sometimes by fraud or error substituted for the latter, it yet has a definite value as a gem, and is sold under the name of spinel ruby or some of its varieties. This value is usually reckoned at about half that of the corundum ruby, although variations in quality of the stones, as well as changes in demand, cause differences of price.
Thus, Emanuel mentions a spinel ruby of good quality, weighing 40 carats, which in 1856 was sold for $2,000, but which in 1862 brought at public auction only $400. In 1866, however, it was again sold for $1,200. The spinel ruby of the French crown jewels, weighing 56 carats, was in 1791 valued at $10,000. Not only is spinel ruby related to corundum ruby in color and use, but the two are frequently
associated together in nature. The gem gravels of Ceylon, Siam, Australia, and Brazil contain both kinds of rubies, and the ruby mines of Upper Burma, where the corundum ruby occurs in crystalline limestone produce also large quantities of spinel rubies. Spinel rubies also come in large quantity from Badakhshan in Afghanistan, near the river Oxus, the name of ballast rubies by which they are often known being said to be derived from Baluchistan or Badakhshan. The Persians have a
tradition regarding these mines that they were disclosed by an earthquake which rent the mountain in Twain. The localities above mentioned furnish nearly all the spinel rubies of commerce. A few have been found in North America, at Hamburg, New Jersey, and San Luis Obispo, California. But these localities have never afforded any appreciable supply. No spinel rubies of great size are known. Bauer mentions as the largest known two cut stones, one of 81 carats and the
other 72½ carats exhibited at the London Exposition of 1862. The King of Aude is said at one time to have possessed a spinel ruby the size of a pigeon's egg. Spinel occurs in many other colors besides red, such as orange, green, blue, and indigo, as well as white and black. Occasionally, colorless spinels occur, and they cannot be distinguished by their behavior in polarized light from the diamond, it is sometimes sought to substitute them for the latter. They can be detected at once,
however, by their inferior hardness. While spinels of any color, if transparent and free from flaws, make desirable gems, the only colors found in sufficient quantity outside of the red to make an appreciable supply are the blue and the black. The blue spinels resemble the sapphire in color, though they are somewhat paler. They come chiefly from Ceylon and Burma, where they occur together with the ruby spinel. The black spinel is known as Ceylonite or Pleonast and is also obtained
chiefly from Ceylon, although occurring of quality suitable for cutting at Mount Vesuvius in Italy. Like the ruby, spinel can be made artificially, the process being to heat a mixture of alumina and magnesia with boracic acid, and if the red color is desired, a little chromium oxide. The spinel ruby seems to have been known to the ancients equally with the corundum ruby, and the two were probably often confounded. The natives of India called the spinel the
pomegranate ruby, and believe to this day that it possesses valuable medicinal properties. Oliver Cummings Farrington. End of section 9.
