Corundum

“ Corundum occurs in pockets, seams, sand veins, narrow streaks, and detached crystals, seldom in large mass. Chester County, Pennsylvania, is apparently the only locality where large, solid masses have been found. The largest annual product of American corundum was six hundred and forty-five tons. Unlike corundum, emery consolidates in large masses. It does not, indeed, form continuous beds of great extent, but its discontinuous masses and.veins sometimes contain hundreds of tons. ”

“ Corundum is next in hardness (though far inferior) to the diamond, and a hard granular form of it is used in grinding and polishing. In its pure, transparent form it is, according to its color, the ruby, the sapphire, the Oriental amethyst, or the Oriental topaz.
The Latin word ruber means red, and the crystalline corundum that is a ruby takes shades from pale rose-pink to a deep crimson that borders on the purple. The color is determined by the nature of the oxide, and the gem sometimes has a light silken sheen. A flawless deep red ruby is one of the rarest and most costly of gems. ”

“ Thus, corundum when pure is colourless, but the presence of traces of certain mineral substances imparts to it not only the red of ruby and the blue of sapphire, but almost every other colour. The tinctorial matter may be distributed either uniformly throughout the stone or in regular zones, or in quite irregular patches. A tourmaline, for instance, may be red at one end of a prismatic crystal and green at the other extremity, or the colour may be so disposed that in transverse section the centre will be red and the outer zone green. A beryl may be yellow and green in the same crystal. ”

“ 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,183 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. ”

“ Diamond and Corundum. A chemist would call a true mineral a pure substance, just as sugar and salt are pure substances to the chemist. Thus diamond is a "mineral species," as is also corundum. There are many different colors of both diamond and corundum, but these different colors are believed to be due to the presence in the pure substance of impurities in small amounts. Thus every diamond consists mainly of pure carbon, and all the corundum gems (ruby and the various colors of sapphire) consist mainly of pure oxide of aluminum. ”

“ In some asteriated corundum (see Asteria) the star is imperfect and may be reduced to a luminous zone, producing an indistinct cat’s-eye effect. According to the colour of the corundum the stone is known as sapphire cat’s-eye, ruby cat’s-eye, topaz cat’s-eye, &c. ”

“ All Scientific Stones Are Corundum Gems. Now the fact that all true scientific stones are corundum gems makes their determination fairly simple on the following basis: Among the considerable number of corundum gems of nature, whether ruby or sapphire of various colors, there is seldom found one that is entirely free from defects. Almost always, even in what are regarded as fine specimens, one will easily find with a glass, defects in the crystallization. ”

“ The various yellow varieties of corundum take the name of the "oriental topaz," which, like most, if not all, the corundum varieties, is harder than the gem which bears the same name, minus the prefix "oriental." Then we have the "amethyst" sapphire, which varies from a red to a blue purple, being richer in colour than the ordinary amethyst, which is a form of violet-coloured quartz, but the corundum variety, which, like its companions, is called the "oriental" amethyst, is both rarer and more precious. A very rare and extremely beautiful green variety is called the oriental emerald. ”

“ But it is doubtful if fine gems are found in this formation, for the conditions which deposited the corundum here, and the more perfect specimens in the true gem strata elsewhere, are quite different.
The colors of the North Carolina corundum are often very fine, and we have seen specimens of a superb blue that retain their hues by candlelight. None of the reds we have ever seen have the true pigeons’-blood tint, but are tinged with blue, and are [Pg 390] therefore of a finer shade when seen by artificial light than by daylight. ”

“ The result thus far, however, is most encouraging in the rock-strata itself, which is the proper gangue of the corundum. With the hundred tons the mine has yielded for abrasive purposes, the workmen have taken from the place of their birth—a solid, undisturbed matrix of ripidolite—beautiful specimens of the nine corundum gems known by lapidaries by the prefix "Oriental," because of their superior hardness and brilliancy ”

“ Grinding pebbles are derived from the erosion of limestone or chalk formations which contain concretions of extremely fine-grained and dense chert. Under stream and wave action they are rounded and polished. The principal sources are ocean beaches.
Corundum as an abrasive is the mineral of this name—made up of anhydrous aluminum oxide. Emery is an intimate mechanical mixture of corundum, magnetite, and sometimes spinel. Corundum is a product of contact metamorphism and also a result of direct crystallization from molten magma. ”

“ The mineral, from whatever locality it comes, is now known in science and commerce as corundum—the name given it by the Hindoos, and meaning cinnamon-stone, from the resemblance in color to that article, of the variety found in their country. It is pure crystallized clay or alumina, and is the next hardest substance in Nature to the diamond, reducing to powder all substances save that gem. ”

“ This is an important point about which mistakes are often made. We must not, for instance, suppose that diamond has twice the hardness of apatite. As a matter of fact, the interval between diamond and corundum is immensely greater than that between the latter and talc, the softest of mineral substances. Intermediate degrees of hardness are expressed by fractions. The number 81⁄2 for chrysoberyl means that it scratches topaz as easily as it itself is scratched by corundum. Pyrope garnet is slightly harder than quartz, and its hardness is said therefore to be 71⁄4. ”

“ Corundum from this mine proves to be of excellent quality. Taking sapphire as the standard at 100, the product of the mine has a power of from 90 to 97 as an abrasive, while that of the best emery, the Naxos, numbers from 40 to 57. The veins, five of which have been opened, run northeast and southwest, dip under at an angle of 45°, and are, at the deepest point reached, seven to ten feet wide. There is also remarkable association of other interesting minerals of tourmaline, spinel, zerkon, etc., while the corundum itself shows almost every shade of color from white to black. ”

“ The property of double refraction is not often very distinct, and by means of this peculiarity it is sometimes detected from the spinel.
[Pg 388]
The term corundum is now applied to the coarser and less transparent kinds of the stone, which have been used as a polishing material from time immemorial. The granular variety known as emery is largely mixed with iron ores, and is far inferior to the transparent and purer varieties as an abrading agent. It is always of a blackish or dark-gray hue, and is often mistaken for iron ore. Asia Minor furnishes nearly all of the emery used in the arts. ”

“ Thus it will be seen that corundum is composed almost wholly of alumina,—one of the constituents of common clay, which, when colored by traces of metallic oxides, chrome, etc., produces a greater variety of precious stones of a high rank than any other mineral. ”

“ The properties of all diamonds are practically alike [135] and so are the properties of all the corundum gems whether red (ruby) , blue (sapphire) , yellow (Oriental topaz) , green (Oriental emerald) , or purple (Oriental amethyst) .
Thus all diamonds, of whatever color, belong to the one species, diamond, and in this case the usual custom in naming them agrees with the facts. Similarly all sapphires, of whatever color, belong to the mineral species "corundum." Thus a ruby is a red corundum. ”

“ My purpose in calling attention to this point is simply to suggest that the proper way to begin the study of minerals and rocks with children is to first familiarize them with the elements of which they are composed. The most important thing to be known about any mineral is its chemical composition; and when a child is told that a mineral—corundum, for example—is composed of oxygen and aluminum, he should have a distinct conception of the properties of each of those elements, for otherwise corundum is for him a mere compound of names. ”

“ Still another variety, greyish-black and generally associated with hæmatite iron ore, is called emery, and, when ground in different degrees of fineness, is so well known by its general use as a polishing medium as to need no description. It should, however, be mentioned that amongst the more coarsely ground emery it is no uncommon thing to find minute sapphires, taking sapphires in their broad, commercial meaning, as signifying any variety of corundum, except the red and the emery. The surfaces of crystals of corundum are often clouded or dull, whilst its classification of lustre is vitreous. ”

“ The oriental jacinth, or hyacinth, is a brown-red corundum, which is more stable than the ordinary hyacinth, this latter being a form of zircon; it changes colour on exposure to light, which colour is not restored by subsequent retention in darkness. ”

“ Corundum, the only important native oxide (Al2O3) , occurs in large deposits in southern India and the United States. Although it contains a higher percentage of metal Ores. (52·9%) than any other natural compound, it is not at present employed as an ore, not only because it is so hard as to be crushed with difficulty, but also because its very hardness makes it valuable as an abrasive. Cryolite (AlF3·5NaF) is a double fluoride of aluminium and sodium, which is scarcely known except on the west coast of Greenland. ”

“ Oryctognostic description of the Gellivara Corundum.
When perfectly cleared of the grayish dust of the iron ore in which the crystals are imbedded, their colour is exactly that of common French flint.
The only form in which it has hitherto been found is that of an oblique octahedron, more or less complete, from the size of a pin's head to that of about three lines across. Very often the crystals are so much compressed that they exhibit to the naked eye only two large faces, the lateral ones being then almost imperceptible. ”

“ Corundum was discovered on Laurel Creek in Rabun county in 1871, and was worked there and at Trackrock, Union county, especially between 1880 and 1893, but in later years low prices closed most of the mines. The limestone formations furnished most of the lime for domestic use. Sandstone, ochre, slate, soapstone, graphite are also mined, and lead, zinc, barytes, gypsum and even diamonds have been discovered but not exploited. ”

“ The best and simplest means for those who are acquainted with the structural defects common to natural corundum gems is to seek for such [104] defects in any specimen that is in question, and if no such defects can be found, to be very sceptical as to the naturalness of the specimen, inasmuch as perfect corundum gems are very rare in nature, and when of fine color command exceedingly high prices. No jeweler can afford to risk his reputation for knowledge and for integrity by selling as a natural stone any gem which does not possess the minor defects common to practically all corundum gems. ”

“ These papers are mostly of value to men of scientific pursuits. Our readers will be interested in more detailed information as to this mineral, and the locality where first in the history of the world it is legitimately mined.
Although corundum has been in use, as an abrasive, from an early age, and under various names, it was not until near the commencement of the present century that its localities were found and examined by scholars, and its true place in mineralogy determined. For thousands of years the Chinese had used it, under the name of adamantine spar ”

“ 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. ”

“ Certain minerals, such as magnetite, ilmenite, spinel, corundum, etc., are often found as primary segregations within the mass of igneous rock. These and other minerals, including minerals of tin and tungsten, monazite, tourmaline, rutile, and various precious stones, are characteristically developed in pegmatites, which are known to be igneous rocks, crystallized in the later stages of igneous [Pg 40] intrusion. ”