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PRECIOUS STONES

SAPPHIRE

OCCURRENCE

In Europe, a well-known and often mentioned locality for sapphires is the Iserwiese, the district in which is the source of the Iser River, which drains the Iser Mountains in northern Bohemia. Sapphire, together with zircon, garnet, and iserine, is found here in loose, alluvial material derived from the weathering of granite. The sapphires sometimes occur as small hexagonal prisms, but more often as water-worn grains of various shades of blue and with various degrees of transparency. While pale blue stones are usually cloudy and opaque the darker ones are, as a rule, transparent. Single sapphires of the finest quality are said to have been found here, all, however, of small size; stones over 4 carats in weight are extremely rare. The deposit, never very extensive, has been systematically worked for many years and is now practically exhausted.

Single stones suitable for cutting have also been found in the garnetiferous sands of Meronitz in Bohemia, in the auriferous sands of Ohlapian in Transylvania, of Urals, of Madagascar, of Borneo, and of some other regions. It is, however, unnecessary to give a detailed account of the occurrence at these localities since in each case the stones are present in such small numbers.

COUNTERFEITING

The blue stones, which may be mistaken for, or passed off as sapphire, are cordierite ("water-sapphire"), kyanite (sapparé), blue tourmaline ("indicolite"), blue topaz, and blue spinel. Amongst such stones may perhaps be included also haüynite, blue diamond, and aquamarine, which in some cases may resemble the sapphire. All, however, without exception, differ from the sapphire in density, most of them being considerably lighter and floating in the heaviest liquid, while corundum sinks heavily; spinel and kyanite alone have a density near that of this liquid (sp. gr. = 3.6 g/cm3). With the exception of diamond these stones, too, are all considerably softer than corundum, by which they can easily be scratched; many of them, indeed, may be scratched even by topaz.

Blue tourmaline, moreover, may be distinguished from sapphire by the difference in the tone of its color, which is an indigo-blue. Kyanite again is characterized by the existence of a system of fine rectangular cracks, which are absent in the sapphire, and are due to the presence of perfect cleavages and twinning. The blue of kyanite, however, is very similar to that of sapphire, hence the name sapparé, but its transparency is less perfect. Cordierite is characterized by its very strong dichroïsm, far stronger than that of sapphire. The specific gravity of topaz is its most salient distinguishing feature. Diamond, spinel, and haüynite are singly refracting, and show no dichroïsm; the same is true also for blue glass, but this substance may be recognized also by its softness.

The color of sapphire is easily imitated in glass by adding to the strass a little cobalt oxide, one part of cobalt oxide to seventy or eighty parts of strass giving a very fine sapphire-blue color. The same effect may be produced under certain conditions by the use of iron. Thus, chemical analysis has shown that the beautiful blue color of an antique vase, ornamented with bas-relief in white, preserved in the British Museum, is due not to cobalt but to iron. The blue color of the slag from iron furnaces is also due to the same metal. It is not, however, customary at the present day to use iron for the purpose of coloring glass. White sapphire, diamond, colorless spinel, zircon, topaz, rock-crystal, and phenakite, as well as colorless strass, may each be mistaken the one for the other by the uninitiated. Of these, sapphire, zircon, and spinel sink slowly in the heaviest liquid (sp. gr. = 3.6 g/cm3), while the rest float. Diamond only is capable of scratching leuco-sapphire, while this scratches all the others. Glass, diamond, and spinel are singly refracting, and can thus be distinguished from the other stones mentioned. Taking into account all these differences, it should not be a matter of great difficulty to distinguish a colorless sapphire from the colorless stones it somewhat resembles.

OTHER COLOR-VARIETIES OF PRECIOUS CORUNDUM.

In addition to the true or oriental ruby and the oriental sapphire there are other varieties of transparent corundum, which are distinguished from these and from each other solely by their colors. We have already seen that these varieties are known by the name of some precious stone which they resemble in color with the qualifying prefix "oriental", Thus certain of these color-varieties of corundum are referred to as "oriental aquamarine", "oriental emerald", "oriental chrysolite", "oriental topaz", "oriental hyacinth", and "oriental amethyst". The precious stones from which these varieties take their names are sometimes given the prefix "occidental"; all are softer than corundum and are easily scratched by it. With the exception of zircon (hyacinth), the specific gravity of which is greater than that of corundum, all the "occidental" are lighter than the "oriental" precious stones; while the latter sink heavily in the heaviest liquid (sp. gr. = 3.6 g/cm3) the former float in this, some indeed floating in pure methylene iodide. Very little familiarity with the appearance of "oriental" and "occidental" precious stones enables one to distinguish the former from the latter solely by the difference in luster; and this difference has led to the term "oriental", conveying by its use an impression of great hardness and brilliant luster in the stone to which the term is applied.

None of the color-varieties of precious corundum now under consideration are abundant in nature. They occur as more or less isolated examples, together with ruby and sapphire, at the localities where these precious stones are found, namely, in Burma, Siam, Ceylon, Montana, North Carolina, etc. Together with the ruby and sapphire they are collected from the various deposits, and are cut and mounted in the same manner, as are these stones, so that further comment on this subject is superfluous.

"Oriental aquamarine" is pale bluish-green or greenish-blue in color, and resembles in this respect, and also in transparency, the variety of beryl known as aquamarine. "Oriental aquamarine" sometimes inclines most to green and other times to blue; specimens are also met with of a dark greenish-blue color, a transition shade between the color of the sapphire and that of the aquamarine. Such stones are remarkable for their especially strong dichroïsm. "Oriental emerald" is corundum of a more or less intense green color resembling that of the emerald, another color variety of beryl. While the "oriental emerald" always shows a tinge of yellow, and is thus inferior to the true emerald in purity and depth of color, it surpasses the latter in transparency and luster. So rare is this variety of corundum that its very existence has been doubted, and it has been suggested that the supposed specimens of "oriental emerald" are in reality true emerald or beryl.

This idea, however, is negated by the well-established occurrences of the stone not only in Burma, Siam, and Ceylon, but also in New South Wales, Montana, and at the Culsagee mine in Macon County, North Carolina, where a crystal measuring 100 by 50 by 35 millimeters was once found. On account of its great rarity the "oriental emerald" far surpasses in value the finest sapphires, but falls short of the value attached to the ruby. It is distinguished from the true emerald by its greater hardness and specific gravity, and by the fact that it is much more markedly dichroïc; the two colors shown by the dichroscope are blue and green. This variety of corundum sometimes varies in color according as it is viewed in reflected or transmitted light. Thus a stone from Chantabun, in Siam, appeared in reflected light a deep bottle-green and in transmitted light a bluish-violet color.

"Oriental chrysolite" is of a pale yellowish-green color; its tint is more yellow than that of the last variety of corundum considered, and corresponds very closely to that of chrysolite (olivine) or to pale colored chrysoberyl. It is much common than "oriental emerald". Clear and transparent greenish-yellow chrysoberyl, with no chatoyant luster, is sometimes referred to as "oriental chrysolite"; it is distinguished from chrysolite proper by its much greater hardness.

"Oriental topaz" ("topaz-sapphire", yellow sapphire) is of a pure yellow color. The value of this stone depends upon the particular shade of its color; specimens of a saffron-yellow tinged with red or of a pure citron-yellow are most highly prized. In the majority of stones the color is a pale straw-yellow, or it may incline to green or brown; in the former case it approaches the color of "oriental chrysolite". Precious corundum with a more or less pronounced yellow color is fairly common; the finely colored "oriental topaz", however, is more rare, and being both rare and more beautiful in color than "oriental chrysolite" is more highly prized. The price of stones showing great depth and intensity of color is as high as that of the finest sapphires. A not uncommon fault in these stones is the existence of a peculiar, avanturine-like, glittering appearance, probably due to the presence of small enclosures. The appearance which gives their names to star-ruby and star-sapphire is sometimes seen in "oriental topaz", which is then referred to as "asteriated topaz" or "topaz-cat's-eye". Tavernier states that he saw among the jewels of the Great Mogul an "oriental topaz" of 157 ¾ carats, which he valued at 271,600 francs (£10,777). Another stone of this kind weighing 29 carats was in the possession of the Parisian jeweler, Caire. It was remarkable for the Arabic inscriptions it bore, not engraved merely on the surface but penetrating the whole thickness of the stone, and was probably an Eastern amulet.

"Oriental hyacinth" ("vermeille orientale") varies in color from pale aurora-red to reddish-brown. The presence of a pronounced tinge of yellow or brown makes its color very different from that of the ruby. This color-variety of corundum is not an important one; it sometimes shows the sheen already mentioned as being present in "oriental topaz". Its specific gravity of 4.0 distinguishes it from true hyacinth (zircon), the specific gravity of which is 4.6 to 4.7 g/cm3.

"Oriental amethyst" (violet ruby, "amethyst-sapphire" or purple sapphire) is violet in color and is of more importance than the last-named variety of corundum. Its tint is often of a bright violet-blue, closely resembling the various shades of color of the true amethyst (a variety of quartz). Sometimes, however, its color inclines to rose-red or purple, and when this is the case the stone appears either like certain almandine-garnets or like certain spinels. This stone, therefore, may be of almost any shade of color between the red of the ruby and the blue of the sapphire. It is distinguished from the true amethyst by its strong dichroïsm, which is apparent even to the naked eye. The light which reaches the eye along the axis of the crystal and out by one of the basal planes is of a warm violet color, while that which travels through the crystal in a direction perpendicular to this is pale and almost colorless. This is a point which must be remembered when the stone is cut as a gem, the lapidary arranging that the table is parallel to the basal planes of the crystal, otherwise the stone will appear pale and insignificant.

The "violet ruby", which by daylight always appears more or less red, has a still more pronounced color, and is even more beautiful by candlelight. Caire, a Parisian jeweler, has described such a stone, which was blue like the sapphire by day and of a fine purple-red by artificial light. We may contrast with this the dull gray appearance of the true amethyst in candlelight. The Maltese Cross, shown in Plate III, Fig. 8, is the form of cutting best suited to the "oriental amethyst"; it is cut, however, in all the forms employed for other colored stones including ruby and sapphire. An "oriental amethyst" of a full deep color is worth approximately as much as a good sapphire.

All the varieties of corundum considered are clear and transparent. Cloudy and opaque corundum, when it possesses some beautiful feature, such as a fine color, may be cut as a gem. A case in point is that of adamantine-spar, semi-transparent, hair-brown corundum, the basal planes of some crystals of which show, like star-stones, a beautiful bluish-white sheen. When such a stone is cut in cabochon in this direction it presents an appearance very similar to that of asteriated ruby. China is considered to be the principal locality for adamantine-spar; it is also found at other places together with precious and common corundum.

[ RUBY  1  2  3  4  5  SAPPHIRE  7  8  9  EMERALD  11  12  13  AQUAMARINE  15  ]
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This document is in the public domain.

March, 2011