CORUNDUM: RUBY and SAPPHIRE
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Some of the most beautiful and valuable of precious stones, including the red ruby and the blue sapphire, belong to the mineral species corundum. All such stones are alike in the possession of those physical characters, which essentially define a mineral species. Their appearance, however, owing to the great variety of color displayed, may be very diverse, and thus the species furnishes a large number of gems, each with a distinct and characteristic color, but having the same chemical composition and crystalline form.
Chemically considered, corundum is pure alumina, the oxide of the now much used metal aluminum. The chemical formula Al2O3 by which this oxide is represented, corresponds to 53.2 per cent of metal and 46.8 per cent of oxygen. Chemical analyses of naturally occurring corundum, however, always show the presence of some impurity, the amount of which is smaller the clearer and more transparent is the material used in the analysis. Foreign impurities are sometimes present in large amount, up to 10 per cent or even more, and when this is the case the stone is rendered cloudy and loses its esthetic value. The analysis of natural corundum has demonstrated the presence of iron oxide, silica, and occasionally traces of chromium oxide. The chemical composition of a beautiful transparent red corundum, the so-called "oriental ruby" and of a blue corundum or "oriental sapphire" of equally fine quality, is given below:
It is on the presence of such foreign substances as iron oxide and in part probably on chromium oxide that the variety of color found in this species depends.
Corundum occurs not infrequently in well-developed crystals belonging to the rhombohedral division of the hexagonal system. A series of the more frequently occurring, forms, which are of two different habits, are shown in Figs. a-i. In some a hexagonal prism is more or less largely developed and terminated at the two ends by basal planes perpendicular to the prism planes, faces of the primitive rhombohedron occupying alternate corners. Most of the crystal-faces are smooth; the basal planes, however, bear regular, triangular striations. Crystals with these three forms are shown in Figs. a, b, c. In Figs. a and b the prism predominates, the only difference between the two being the greater size of the rhombohedron faces in Fig. b. In Fig. c the prism faces are narrow, while the rhombohedron faces and the basal planes are all about equal in size. In Fig. d is a combination of the same forms with a double hexagonal pyramid in addition, the twelve faces of which replace the edges between the prism faces and the basal planes. This hexagonal bipyramid is present in all the remaining Figs. e to i. In Fig. e it occurs alone, and in the remaining forms it is predominant; in Fig. f it is in combination with the basal planes, and in Fig. g with the basal plane and a rhombohedron, the faces of which replace alternate corners above and below. In Fig. h there are three such hexagonal bipyramids with different inclinations, each successive pyramid being less steeply inclined the nearer it is to the end of the crystal; in combination with these three pyramids are -the basal planes and a narrow hexagonal prism. The frequent repetition or oscillation between pyramids of this kind gives rise to horizontal striations on the faces, such as is shown in Fig. f. In Fig. i, a combination of two hexagonal bipyramids with the hexagonal prism and an acute rhombohedron is shown. As mentioned above, crystals of corundum occur in one of two habits, that is with either the prism or the hexagonal pyramids predominating. The former is the more characteristic for red corundum or ruby, and the latter for blue corundum or sapphire.
Twin intergrowths are often met with in cloudy corundum, but less frequently in transparent material suitable for use as gems. These are of two kinds: in the one, large numbers of thin plane lamellae, parallel to a face of the primitive rhombohedron, occupy twin positions in the crystal, as indicated in Fig. a; in the other, illustrated by Fig. b of the same figure, the lamellae are parallel to the basal planes. Owing to this cause the faces of crystals often bear fine striations, which differ in direction in the two kinds of twin intergrowth. In the first, the striations are present on the basal plane in three sets of parallel lines intersecting at 120° or 60°, and on the prism and rhombohedron faces in two sets intersecting at an oblique angle. In the other kind of twin intergrowth, the basal planes are smooth, while the faces of the prism and rhombohedron are horizontally striated, the striations on the prism faces being perpendicular to the prism edges. This twin lamination of corundum crystals makes the mineral appear to have two cleavages, one parallel to one or more faces of the primitive rhombohedron and the other parallel to the basal plane. Such a crystal when dropped or struck with a chisel will separate along plane surfaces in these directions. This splitting up, which is known as a platy separation, is not a true cleavage but is due to want of cohesion between the individual lamellae. If the division were due to cleavage the lamellae after separation could be split up again in the same direction, which is not the case. Moreover, the property of splitting along surfaces parallel to the faces of the rhombohedron and to the basal plane is possessed only by those crystals, which are penetrated by twin-lamellae parallel to these faces. In crystals in which twin-lamellae are absent all trace of platy separation is also absent, the fracture being conchoïdal as is the case with many other non-cleavable minerals. Corundum is brittle, and among other physical characters it possesses that of hardness to a high degree. After diamond, it is the hardest of all minerals, having the number 9 assigned to it in Mohs' scale of hardness. It is, however, much more nearly approached in hardness by certain other minerals than it itself approaches diamond in this respect. Its hardness renders this mineral very valuable as a grinding and polishing material, the cloudy and opaque varieties, especially the compact black emery, being much used for this purpose, while rather superior qualities are utilized for the pivot supports of watches and various delicate instruments. There are degrees of hardness among the different varieties of corundum, the blue sapphire standing first in this respect.
The specific gravity of corundum is very high; that of pure material is very near to 4 g/cm3, values varying between 3.94 and 4.08 g/cm3 having from time to time been determined on pure material. Greater variations on either side of the mean value of 4.0 g/cm3 are due either to errors of observation or to the determinations having been made on impure material. No difference has been observed in the specific gravity of differently colored varieties. Corundum is thus one of the densest of precious stones, and this character renders it easily distinguishable from other minerals, which may resemble it in appearance. It sinks rapidly, not only in methylene iodide but also in the heavier liquid (sp. gr. = 3.6 g/cm3) obtained by dissolving, iodoform and iodine in methylene iodide. It is not attacked by acids either in the cold or when warmed, and is completely infusible before the blowpipe. Many specimens of corundum when heated in a dark room display beautiful phosphorescence. When rubbed with cloth or leather the mineral acquires a charge of positive electricity, which it retains for a considerable time.
Corundum varies greatly in appearance. Most frequently it is cloudy and opaque, and only a small proportion is clear and transparent and valuable from an esthetic point of view. The common cloudy and opaque varieties of corundum will not be here considered in detail, but our attention will be directed to the transparent varieties known as noble or precious corundum. The luster of precious corundum is very fine, and is displayed to great advantage by the polished facets of a cut stone. It approaches that of diamond in brilliancy, but differs from it in character, being not adamantine but vitreous, like that of many other precious stones and glass. So brilliant and perfect a luster and so marked a fire is displayed by no other precious stone with the exception of diamond, and perhaps also of colorless hyacinth (zircon), so that by these characters alone the stone may be distinguished from other gems of the same color. Even when a specimen of corundum is of a poor color it will still be cut as a gem, since the fine 1uster of the mineral will redeem the stone from insignificance. Moreover, owing to the great hardness of this stone, the luster it" retained even after rough usage and hard wear.
Corundum is optically uniaxial and doubly refracting. Its refraction, though strong, is considerably less than that of diamond; its double refraction is small, the refractive indices for the ordinary and extraordinary rays differing but slightly. In a crystal from Ceylon they have been determined for yellow sodium light to be: omega = 1.7690 epsylon = 1.7598. The dispersion produced by corundum is also small, the refractive indices for different colors of the spectrum differing but slightly; hence the mineral shows no marked flashes of prismatic colors as does the diamond.
The high index of refraction, small dispersion, and considerable hardness of corundum render the colorless or pale-colored varieties very suitable for the preparation of microscopic lenses. Pritchard who, as we have already seen, made use of diamond for this purpose, in 1827 constructed lenses of very pale blue sapphire; they have never, however, come into general use. Transparent corundum finds its most extensive application in jewelry, the fine appearance of the stone being due to the combination of transparency and high luster with fine color. The mineral corundum includes some of the most beautifully colored precious stones known, and we will now consider the different color varieties of the mineral. Absolutely pure crystallized alumina has no color and is perfectly water-clear. It is then known as leuco-sapphire; it is, however, but rarely found in this condition, more usually showing a more or less pronounced color. In many cases the coloring-matter is irregularly distributed, sometimes being aggregated in patches surrounded by colorless portions of material; not infrequently also different portions of one and the same crystal are differently colored. As has been already stated, the various colors shown by corundum are due to the intermixture with the ground substance of various foreign substances, the nature of which in particular cases has never been determined. The colors vary in tone from light and pale to dark and intense; stones showing pale tones of color have been described as" feminine", and those colored more deeply as "masculine".
The phenomenon of dichroïsm is always observable in deeply colored specimens of corundum, it is not seen in very pale colored varieties, and is the more noticeable the deeper the color of the specimen.
Several varieties of precious corundum are recognized; these are distinguished from each other by their color, and from the jeweler's point of view differ much in importance and value. The two varieties, which occur with the greatest frequency are red corundum, or ruby, and blue corundum, or sapphire, the latter being more abundant than the former. All other colored varieties are in comparison almost rare. They are known by the same names as certain other stones of the same color, being distinguished from these by the prefix "oriental", which is meant to signify the possession of especially noble qualities, such as great hardness and fine luster. The same prefix is often applied to ruby and sapphire themselves with the object of distinguishing them from other precious stones of the same color. Moreover, in the case of sapphire, it serves to distinguish stones of the true sapphire-blue from other color-varieties of the species, to which, with the exception of ruby, the term sapphire is sometimes extended. It should be remembered that the only character which the different color-varieties of corundum have in common with the stones after which they are named is their color; thus, by the term oriental emerald must be understood transparent corundum of a green color.
The different color-varieties of corundum are tabulated below:
The different colors of these varieties of corundum usually appear just as beautiful when viewed by artificial light as by daylight, which is not always the case with the stones, after which they are named. The color of certain varieties of corundum can be in some cases destroyed, and in others changed by the application of heat; this point, however, will be considered later on.
The different color-varieties of corundum occur as irregular grains and as well developed crystals embedded in the mother-rock, which, as a rule, is an old crystalline rock, such as granite or gneiss. The gem-varieties are found with especial frequency as secondary contact minerals, which have been developed in limestone by contact with a molten igneous rock. By the weathering and denudation of such rocks, the embedded crystals are set free, and are subsequently found with other water-worn debris in the beds of streams and rivers. It is in these derived deposits that the most beautiful specimens of the varieties mentioned above are found in all countries in which the original mother-rocks occur.
Having briefly considered the characters common to all corundum, we pass now to the consideration of the varieties suitable for use as gems, and begin with the most costly of all stones, namely, the ruby.
Diamond Diamond Geology and Mining Diamond Trade
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