Whenever possible, I provide, in parentheses, today's equivalent prices in US dollars. These prices take account of inflation and are based on available historical exchange rates. The inflation rate is calculated on the assumption that 1 oz. of gold has always the same value; only due to a loss, with time, of currency value it takes more money to buy the same 1 oz. of gold.
Sapphire ("oriental sapphire") is the name given to blue corundum. Sapphire differs from ruby most essentially in color, it is in addition, however, slightly harder, being the hardest of all the varieties of corundum; and, moreover, is stated to have a slightly higher specific gravity, the specific gravity of ruby being given as 3.99 to 4.06 g/cm3, and that of sapphire as 4.08 g/cm3. The form of a crystal of sapphire agrees completely in its general symmetry with that of a crystal of ruby; the two crystals differ somewhat in habit however. The prism and rhombohedron, usually well developed in the ruby, are subordinate in the sapphire, and here the hexagonal bipyramid predominates, as is shown in Fig. e-i. While the ruby is usually colored uniformly throughout its substance, the distribution of color in the sapphire is often very irregular. A single stone may show an alternation of colorless, pale yellowish, and blue portions, or in the colorless groundmass of a stone there may be patches of a blue color. Such stones, compared with a sapphire of a uniform blue color, are almost worthless as gems.
It would be quite possible to collect a series of sapphires showing small gradations in color from deep blue to yellowish or colorless stones. These latter are known as white sapphire (leuco-sapphire); they are only rarely perfectly colorless and transparent, usually showing a bluish or yellowish tinge. Sapphires of a definite yellow color are described as "oriental topaz".
The blue color of the sapphire disappears on heating, hence it is possible to transform a patchy or pale-colored stone into a leuco-sapphire, and at the same time greatly enhance its value.
The distribution of the blue patches in the colorless or yellowish groundmass of a sapphire is usually quite irregular, only occasionally is there any definite arrangement to be observed. In such cases, the crystal may be blue at one end and colorless at the other, or the middle portion may be colorless and the two ends blue, or colorless and blue bands may alternate. Moreover, different portions of the same specimen may exhibit different shades of blue, such as pure blue and the greenish-blue peculiar to the sapphires of Siam, or even colors, which are altogether different. Thus, crystals of sapphire are known which are blue at one end and red at the other, and others of which the two ends are blue and the middle part yellow. A crystal answering to this latter description and weighing19 1/8 carats was exhibited in the mineralogical collection in the museum of the Jardin des Plantes in Paris, France.
The peculiar distribution of color in such sapphires has been sometimes ingeniously utilized; for example, the figure of Confucius, once described, was carved out of a partly colored sapphire in such a way that the head of the figure is colorless, the legs yellow, and the body pale blue. Every shade of blue, from the palest to the darkest, is represented amongst sapphires. The very dark shade of blue, closely approaching black, is described as inky; very pale "feminine" sapphires are sometimes described as "water-sapphires", while stones of the darkest shade are variously known as "indigo-sapphire", "lynx-sapphire" or "cat-sapphire". So long as the depth of its color does not interfere with the transparency of a sapphire, the darker it is in color the more highly is it prized. The color of sapphires shows as great a variety of tone as of shade. Thus, we have stones of indigo-blue, cornflower-blue, grayish-blue, and greenish-blue, the last being especially common. The most admired tone of color for a sapphire is an intense cornflower-blue. A really fine sapphire will combine with this color a beautiful velvety luster; the latter character, though occasionally seen, is by no means common.
The blue of the sapphire is always more or less tinged with green; this being very noticeable when one looks through the stone in certain directions. Like the ruby the sapphire is distinctly dichroïc, the phenomenon being very marked in dark colored stones but scarcely noticeable in stones of a light shade of color. Looked at in the direction of the optic axis, that is to say, along the line joining the apices of the hexagonal bipyramid or perpendicular to the terminal basal planes, a crystal of sapphire appears of a pure blue color, more or less intense or inclined to violet, according to the particular character of the stone. In a direction perpendicular to this the stone appears paler, and its blue color is distinctly tinged with green; observed in intermediate directions the sapphire will appear of intermediate tints. The dichroïsm of the sapphires of Siam, which have come into the market in large numbers, as well as of those from Le Puy in Auvergne, and from some other localities, is especially well marked.
If the light passing through a crystal of sapphire in the direction of its optic axis be received in a dichroscope, the two images formed by the instrument will be identical in color either pure blue or blue tinged with violet-and the color will remain unchanged when the stone or the instrument is rotated. If examined in the direction perpendicular to this, the two images will, as a rule, be colored differently. In the position in which the greatest difference in color exists, the one will be of a pure dark blue and the other usually of a paler greenish-blue, but sometimes of a yellowish-green. It follows from these facts that the pure blue color of a sapphire crystal is best displayed in the cut stone when the table of the latter is perpendicular to the optic axis, and parallel to the basal planes, of the crystal. It will be remembered that it is advantageous for the same reason to cut crystals of ruby also in this manner.
The appearance of sapphire in artificial illumination varies in different specimens. In some no difference in color can be detected, in others the color becomes darker, or it may change to reddish, purple, or violet. The latter change in color is rare, and stones, which show it are valuable on this account.
While the color of the ruby remains unaltered after the stone has been exposed to a strong red heat that of the sapphire under similar conditions disappears, although in other respects the stone remains unchanged. When exposed to very high temperatures, however, the sapphire, like the ruby, becomes gray and cloudy. The depolarization of the sapphire does not take place with equal facility in all stones. Indian sapphires lose their color most easily and there are stones the color of which it is impossible to completely destroy. From the fact that sapphires can be decolorized by heat, it has been argued that their color is due to some organic compound. Some authorities have referred it to the presence of a small amount of iron, which has been detected in analysis, while others, relying on Fremy's experiments in the artificial production of rubies, considered it to be due to small quantities of some compound of chromium.
There is often to be seen on the basal planes of sapphire crystals a six-rayed star of chatoyant light. This appearance, which is known as asterism, and is often very beautiful, is best displayed by cutting the crystal in which it exists en cabochon, the center of the curved surface lying in the axis of the crystal. The rays of the star spread out to the margin of the stone, and the movements of the latter are followed to a certain extent by the star, the center of which is always directed towards the light. A perfect six-rayed star is, however, seen less frequently than a patch of chatoyant light, which may be more or less rounded or elongated in outline, in the latter case being regarded as a single ray of the star. This patch or star of milky, shimmering, and opalescent light is sometimes tinged with a red or blue color. The rays of the star may be narrow and sharply defined, showing up against the dark surface of the stone like silver threads, or they may be broad and mal-defined, merging imperceptibly into the darker portions of the stone. A sapphire in which the star is sharply defined, is much prized, and is known variously as a star-sapphire, or asteria or star-stone, or asteriated sapphire, or as a sapphire-star-stone. A stone, which shows only an irregular patch of opalescent light, is known as sapphire-cat's-eye, "oriental girasol", or opalescent sapphire. To rubies showing the same appearance corresponding names are applied, namely, star-ruby, asteriated ruby, ruby-star-stone, ruby-cat's-eye, and opalescent ruby. The phenomenon of asterism is not confined entirely to red and blue corundum, but is also occasionally seen in yellow corundum or "oriental topaz". A stone of this kind is, like ruby-cat's-eye, included in the term "oriental girasol" when it shows an elongated or round patch of opalescent light; it is then called a "topaz-cat's-eye", and when it shows a regular star it is called a "star-topaz".
No exceptional value is attached to asteriated stones; a fine star-sapphire is about equal in value to an equally fine stone of the ordinary kind, though large star-rubies fetch rather higher prices than ordinary stones; small one can be obtained for comparatively little. None of the asteriated varieties of corundum are confined to any special locality, being found wherever precious corundum is found.
The phenomenon of asterism has been variously explained. Some consider it to be due to the reflection of light from the surface of the twin-lame lire, which are present in such crystals in large numbers, and are arranged so that their planes are parallel to the faces of the primitive rhombohedron (Fig. a). The existence of these lamellae is indicated by the striations on the basal planes. These striations are grouped in three sets inclined to each other at angles of 60°, and were considered by Babinet to be the cause of the star of opalescent light. A more probable explanation is that it is due to the reflection of light from the surface of a multitude of microscopically small tubular crystals of rutile or tourmaline enclosed in the crystal, and grouped into three sets, each of which is parallel to a face of the hexagonal prism; corundum crystals also enclose minute tabular crystals, consisting of alteration products of the corundum, and arranged, like the rifts, in three sets inclined at 60° to each other. The six rays of the star are produced by the total reflection of light from the surfaces of these tubular cavities, and perhaps also from the surfaces of the tabular crystals. The phenomenon is only to be observed in stones in which there are large numbers of these enclosures, such stones being usually cloudy and having a metallic sheen. Star-stones are never in fact quite clear and transparent throughout their whole mass; frequently also they are built of alternate blue and colorless layers. The same phenomenon may be observed in many opaque specimens of common corundum, especially in the brown adamantine-spar, which is sometimes cut so as to show the asterism. Enclosures of the kind described above are more frequent in blue sapphire than in corundum of any other color, and, as a consequence, star-sapphires are commoner than other varieties of asteriated corundum.
In Europe star-sapphires only, as a rule, are cut en cabochon; in India this form of cutting is much more frequently employed, not only for star-stones but also for others. Such stones, however, on their arrival in Europe are always recut with facets. Both the form and methods of cutting employed for the sapphire are identical with those used for the ruby, as is also the mounting of the stone. The color of the sapphire is frequently intensified by placing a piece of blue silver-foil beneath the stone in its setting.
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