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PRECIOUS STONES

AQUAMARINE "AQUAMARINE-CHRYSOLITE" GOLDEN BERYL

The occurrence of beryl in the gold-washings of the Sanarka River in the Southern Urals is also of small importance; here the mineral is found as pebbles associated with topaz, chrysoberyl, etc.

The beryl of the Altai Mountains is distinguished less for the beauty of its crystals than for their size, prisms with a length of 1 metre and a thickness of 15 centimetres being met with. These crystals, which have the usual form, namely, a hexagonal prism terminated by a basal plane perpendicular to the prism planes, range from sky-blue to greenish-blue in colour, and occur in brown, much fissured quartz, the exact locality being a spot in the Tigirezh Mountains. The mineral is here at the best only translucent, and therefore rarely of use as a gem.

Of greater importance is the occurrence of beryl in the Nerchinsk district of the province of Transbaikalia in south-east Siberia, Nerchinsk itself being in longitude 116° E. of Greenwich on the upper course of the Shilka River, a tributary of the Amur. There are here two stretches of country in which beryl, and especially aquamarine, abounds, the one being the mountain range Adun-Chalon and its southern continuation, the mountains of Kuchuserken, and the other the neighbourhood of the Urulga river on the northern side of the Borshchovochnoi Mountains.

The variously coloured precious stones, which occur at Adun-Chalon (Adun-Tschilon), have been known since the year 1723. The output of gems from these deposits was formerly very considerable; it reached its highest in the year 1796, when no less than 70 kg. of pure aquamarine, suitable for cutting as gems, was obtained. The crystals of beryl are found here attached to the wa11s of cavities in a topaz-rock, which consists mainly of finely granular quartz and small topaz crystals, and occurs as veins penetrating the granite. The aquamarine in these cavities is accompanied by topaz and smoky quartz, frequently also by other minerals. The highest mountain of the Adun-Chalon range has two peaks, separated by a narrow va11ey. The western peak is known as Hoppevskaya Gora, that is to say, Schörl Mountain; it consists almost entirely of topaz-rock, and is scarred from foot to summit with the workings of gem-seekers. The mineral is by no means, however, confined to this mountain, numerous mines being scattered about an area of one square mile in the neighbourhood. These mines are nothing but open pits or trenches of the most primitive kind, without timbering, and never more than three fathoms in depth; from these, short tunnels are worked in the rock in all directions. Immediately beneath the turf covering the southern slopes of the Hoppevskaya Gora is a layer of loose material, containing much iron-ochre, derived from the weathering of the topaz-rock. In this layer fine specimens of aquamarine, and its customary associate topaz, are to be found. A hexagonal prism of transparent beryl, 31 centimetres (over 1 foot) in length and 5 centimetres in diameter, from Adun-Chalon, is preserved in the British Museum collection of minerals.

The beryls of Adun-Chalon differ from the smooth-faced prisms of the Urals and of the Borshchovochnoi Mountains (or Urulga River) in that the prism-faces are deeply striated (Fig. d). The crystals are, as a rule, greenish-blue in colour, but sky-blue, yel1owish-green, wine-ye11ow, and colourless specimens are met with; and every degree of transparency is represented. The crystals are often united in groups, which are frequently invested with a thin surface layer of iron-ochre, the substance with which the drusy cavities are filled. The country between the rivers Shilka and Unda in the Borshchovochnoi Mountains abounds with fine beryl. A large amount of the mineral was obtained about the middle of the nineteenth century, for the most part from the granite mountains which border the Urulga River, a tributary of the Shilka on its right bank. Beryls from the neighbourhood of the Urulga are remarkable for their size, transparency, and beautiful colour. The majority are yellowish-green, the remainder being variously tinted or colourless. The crystals may reach a length of 10 centimetres and a thickness half as great; they are frequently developed with great regularity. Beryl from the Urulga River is in general very similar to that from Mursinka in the Urals.

In other parts of Asia precious beryl occurs but sparingly. Aquamarine has been found at some places in India, and various objects worked in this mineral have not infrequently been found in ancient tombs, temples, etc. Most of it appears to have been obtained in the Coimbatore district of the Madras Presidency, as at Paddur or Patialey, where, at the beginning of the nineteenth century, the mineral was obtained from cavities in coarse-grained granite. When all the more easily obtained stones had been taken, work was abandoned. Later on, aquamarine was discovered at Kangayam in the same district; specimens from this locality were shown at the Vienna Exhibition of 1873, and others are preserved in the British Museum. Here was once found a stone of the most perfect transparency, which weighed 184 grains (900 carats) and sold for £500 ($54,327).

Pale blue crystals of fair size, sometimes measuring as much as 31 inches in length, are found at many places in the Punjab in granite veins penetrating gneiss. They are, however, almost invariably much fissured and unsuitable for gems. In the Jaipur State in Hajputana aquamarine is mined in the neighbourhood of Toda Rai Sing in the Ajmer district, in the Tonk Hills, and at various places lying within a radius of 38 miles from Rajmahal on the Banas River. Most of these crystals are quite small and therefore, in spite of their fine colour, of little value. They are found buried in marshy ground, and have probably been derived from the granite veins, which penetrate the sedimentary rocks of Rajputaua in large numbers. Small crystals of yellow beryl occur embedded in a thick vein in the Hazaribagh district in Bengal. Other reputed Indian localities require authenticating.

In Burma, pebbles of aquamarine are reported to have been found in the Irrawaddy. Whether this is truth or not, it is certain that beryl is of only sparing occurrence in Burma; while in Ceylon, a locality so rich in other precious stones, it is practically non-existent. Although in Europe many localities for common beryl are known, precious beryl of gem quality occurs but sparingly. In the Mourne Mountains, in County Down, Ireland, crystals of aquamarine of a beautiful and comparatively deep blue colour occur, together with topaz, in cavities in granite; these, however, are rarely perfectly transparent.

In the United States of North America numerous localities are known, from which tine stones of various colours, and of a quality suitable for cutting, have been obtained. The mineral is found, for example, with the emerald in Alexander County, North Carolina, while at Russell Gap Road, in the same county, more aquamarine of gem quality was found than anywhere else in the United States. Fine blue aquamarine is found also in Mitchell County, North Carolina, and green beryl at Stoneham, in Oxford County, Maine; a fine bluish-green fragment, found recently at the latter place, gave an almost faultless brilliant, weighing 1,331 carats, and measuring 35 millimetres in length and breadth and 20 millimetres in thickness. Golden-yellow beryl of good quality is found at Albany in Maine, in Coosa County in Alabama, and at a few other places. At Royalston in Massachusetts there occur, with other varieties of precious beryl, some of a fine blue colour comparable to the blue of the sapphire; it is by far the most beautiful blue beryl known, but, unfortunately, occurs only in quite small crystals. Beryl is also found in Colorado, namely, on Mount Antero, ten miles north of Salida, at a height of 12,000 to 14,000 feet above sea-level. The crystals, which range in colour from a pale to a dark shade of blue, are found, together with phenakite and other minerals, attached to the walls of drusy cavities in granite. They vary in length from 1 to 4 inches, and in thickness from to inch to an inch; from the largest a faceted stone of about 5 carats can be cut. There are many other localities in America at which beryl is found, but none of any commercial importance.

A small amount of beryl occurs also in Australia; at several places in New South Wales for example. Here, again, the occurrence has no economic significance. Precious beryl of a yellow colour, and also the yellowish-green "aquamarine-chrysolite," come principally from Brazil, although it is to be found in good quality at some of the localities already mentioned, for example, in Siberia associated with aquamarine. Beryl of a deep, pure yellow is known as golden beryl. It occurs at many beryl localities in North America, especially at Albany in Maine; it has been collected also in the vicinity of New York City, and in Litchfield County, Connecticut. It is always of sparing occurrence in the States, and, though highly prized there, does not in general command high prices, only exceptionally fine stones costing more than a few dollars per carat.

Certain of the several varieties of precious beryl are liable to be mistaken for other precious stones, which they resemble in appearance; the exceptionally low specific gravity of beryl, however, prevents any serious confusion. Aquamarine resembles in colour "oriental aquamarine," euc1ase, some tourmalines, and blue topaz; its resemblance to blue topaz is so close that the latter is often known in the trade as aquamarine. Each of the four stones mentioned above, however, sinks in liquid No. 3 (sp. gr. = 3.0 g/cm3), while beryl floats. In the same way yellow beryl, that is to say, "aquamarine-chrysolite" and golden beryl, may be distinguished from other yellow and greenish-yellow stones of similar appearance, namely, from yellow topaz, "oriental topaz," "oriental chrysolite", chrysolite, and chrysoberyl, all of which sink in liquid No.3. To distinguish between yellow beryl and yellow quartz (citrine) is less easy, for there is no great difference between the hardness and specific gravity of these two minerals. In liquid no. 4 (sp. gr. = 2.65 g/cm3) citrine remains suspended while beryl slowly sinks; moreover, a smooth surface of quartz will be untouched by citrine, but will be distinctly, though not deeply, scratched by beryl. The stronger dichroïsm of beryl may also serve sometimes to distinguish it from citrine.

A glass resembling aquamarine in colour may be obtained by fusing together 3456 parts of strass, 24 parts of glass of antimony, and l 1/2 parts of cobalt oxide. The single refraction, entire absence of dichroïsm, and low degree of hardness of this imitation, are the characters whereby it is distinguished from genuine aquamarine.


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[ 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