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PRECIOUS STONES

EMERALD

The emeralds are usually of the finest dark green colour, but paler or almost colourless stones are also found, as well as crystals, which are quite black, the latter being especially remarkable for their velvety lustre. Occasionally crystals are found which are green on the exterior but colourless inside. The crystals are classified for trade purposes according to transparency and depth of colour. Almost all have the simple form of a six-sided prism with basal plane (Figs. 62 a, and 63). They rarely exceed the size of a man's thumb, and are usually smaller. Frequently they are broken across in one or more places, the cracks being filled up with thin layers of calcite, so that as long as the crystal remains in its matrix it appears whole and unbroken, the fracture only becoming evident when the crystal is detached from the matrix. Together with the well-formed crystals are found rounded fragments of emerald, a fact, which affords a certain support to the theory that the calcite veins are not the primary situation of the emeralds, but that they have been washed into these veins from gneissic or granitic rocks.

Some crystals have the peculiarity of falling to pieces, with no apparent cause, after being taken from the mine. It has been sought to avoid this by placing the emeralds when first unearthed in a closed box, thus protecting them for a few days from the action of light and allowing them to dry slowly. This device is not, however, as a rule, successful. Moreover, most of the emeralds which come from the mine clear, transparent, and free from fissures, in course of time lose their transparency and assume the usual turbidity of this stone owing to the development of fissures within them. In this connection we may recall the statement that emeralds, both from this and from other localities, only acquire their own particular hardness after they have been taken from the mine for some time. Fine emeralds suitable for use as gems are known in Colombia as "canutillos" and poorer stones as "morallion".

Since there is nothing to indicate in what part of the mine emeralds are likely to occur, the workers simply loosen blocks of rock from any part of the walls until a nest of emeralds, the presence of which is indicated by green quartz crystals, is met with. This is then carefully broken out and taken away. The material loosened from the walls of the mine is allowed to fall to the bottom, and when it has accumulated to a certain extent it is washed out into a canal constructed for this purpose, by the sudden fall of a head of water stored on the heights above the mine. The canal empties itself into the Minero, which carries the mine debris still further away. This was the method adopted later, but in former times when the Spaniards were in possession the workings were all underground.

The only other locality of importance for emeralds beside Colombia is the Ural Mountains. There is here only one mine from which emeralds are obtained, the same in which Uralian chrysoberyl, that is to say, alexandrite, is found. It is situated on the right bank of the Takovaya, a tributary stream of the Bolshoi Reft (that is to say, Great Reft), which flows into the Pyshma River, about fifty-seven miles east of Ekaterinburg. The finest of the Uralian emeralds are quite equal in transparency and beauty of colour to South American stone~. In this, as in other localities, perfect crystals are rare, the majority being fissured and opaque, and the colour, in many cases, being irregularly distributed or too pale. The crystals commonly have the form of a hexagonal prism, often terminated irregularly, but sometimes with a basal plane like Colombian crystals; other forms scarcely ever occur. In size Uralian emeralds often exceed those from other localities, especially South America. Some of exceptional sizes have already been mentioned; the largest have a length of 40 centimetres (15 3/4 inches), and a thickness of 25 centimetres, but they are not, as a rule, of good quality.

The mode of occurrence of the emerald in the Urals is similar to that in Egypt, but differs from that in Colombia. The stones are found embedded in a mica-schist, which is interfoliated with chlorite-schist; this is also the mode of occurrence at Habachthal in the Salzburg Alps. Scales of mica are found on the surface of, or enclosed in, emerald crystals from all these localities. Uralian crystals occur singly or in groups; they are often grown together in parallel position, but occasionally radial aggregates of columnar crystals are met with. The emerald locality on the Takovaya was discovered accidentally in 1830, a peasant noticing a few small green stones among the roots of a tree torn up by the wind. These stones were picked up and taken to Ekaterinburg, where the gem-cutting works of the Tsarina, Catharine II, had been established as far back as 1755, and where the many beautiful stones found in the Urals were and are still worked. After this the locality was carefully examined, and a mine opened in the mica-schist. With the emeralds found in this rock are associated alexandrite, phenakite, apatite, rutile, fluorspar, and other minerals, besides another variety of beryl, the pale-coloured aquamarine. Emerald was at first comparatively abundant at this locality, the yield, however, has gradually fallen both in quality and quantity, and the mines are by no means in full work. They have in 1900 rented by a British company; the venture, owing mainly to theft of stones, has not, however, been successful.

This is the only spot in the Urals at which emeralds occur in large numbers. Only once has a finely coloured and transparent stone been found elsewhere, namely, in the gold sands in the valley of the stream Shemeika, in the Ekaterinburg mining district. The emeralds reported by the ancients to come from Scythian lands may actually have been found in the Urals, but nothing is exactly known as to their origin.

The occurrence of emerald in the Salzburg Alps is similar to that at Takovaya; the crystals found at the former locality are, however, smaller, and their lustre less brilliant, so that from a trade point of view they are unimportant. The spot at which they are found lies above the Sedlalp (or Söllalp) on a steep wall of rock, the "Smaragd-Palfen", on the east slopes of the Legbach ravine, a side branch of the Habachthal. It is 7,500 feet above sea level and very inaccessible. The deposit is not rich enough to justify any extensive workings; the stones have been mined by irregular methods and at the risk of the workers' lives for a long period, it is said since the time of the Romans.

Here also the form taken by the emerald crystals is that of a hexagonal prism, and on the surface of the crystals scales of mica and needles of black tourmaline are often to be seen. In colour the crystals are sometimes of a fine, dark emerald-green, but more often of a pale grass-green or greenish-white; moreover, the colour is frequently irregularly distributed. Perfectly transparent crystals are rare; the majority are turbid, semi-transparent, or translucent to opaque, only a small proportion being fit for use as gems. The white or pale-coloured crystals are usually larger and purer than the green ones. The crystals vary from a line to an inch in length and from 1 to 3 lines in thickness; stones exceeding these dimensions are exceptional. The mother-rock in which the crystals are embedded is a finely granular mica-schist, dark-brown or greenish in colour and resembling clay-slate; it is interfoliated with a green mica-schist, which is rich, in some places, in chlorite, and in others in hornblende. Iron-pyrites occurs in association with the emeralds. The finest and largest stones are said to be found in comparatively thin veins of mica with a thickness of about 1 to 3 inches.

Besides the locality described above, there are a few other places in the neighbourhood at which emeralds are found; none, however, are of any importance. Among other European emerald localities we may briefly mention Eidsvold, on the southern end of Mjösen Lake in Norway. The crystals, which are here embedded in granite, are nearly all turbid and pale in colour, and are therefore not, as a rule, cut as gems.

All emerald localities other than those, which have been mentioned, are unimportant, and it would seem that in some supposed localities the occurrence of emerald is doubtful. Thus, for example, it appears that there has been no well-authenticated occurrence of emerald in India or Burma, although both countries are often described as emerald localities. The green stones, the occurrence of which in Rajputana, in northwest India, is fairly authentic, may very possibly be chrysoberyl. The emerald is a stone, which is highly prized in India, but the emeralds now in the country were probably brought either from South America or perhaps from the Urals. This is probably the history of the emeralds, which are in the collection of jewellery in the Indian Section of the Victoria and Albert Museum, South Kensington. Among the Burmese regalia there exhibited is a fine slice, about 2 inches across, of a large hexagonal crystal of emerald. A large number of emeralds were sent from London to India, and it is also stated that South American stones were sent direct to India to be cut after the manner customary in that country, after which they are placed on the market as stones of Indian origin.

Emerald is said to occur as pebbles in Algeria, namely, in the Harrach and Bouman rivers, and also in situ in the neighbourhood, but according to other statements the mineral in question is green tourmaline.

The emerald localities in Australia are not important. Mount Remarkable in South Australia is one, and there are a few in New South Wales. At one of these, nine miles northeast of the township of Emmaville in the County of Gough, NSW, mining operations, first for tin and then for emerald, have been carried on, not apparently, however, with very successful results. The emeralds are found here in a pegmatite vein, which is an offshoot from a mass of granite penetrating clay-slates, probably of Carboniferous age. The associated minerals are topaz, fluorspar, cassiterite, and mispickel. The colour of these emeralds ranges from a pale shade of green to a moderately bright grass- or emerald-green; the crystals never have any great depth of colour, and they resemble beryl almost more closely than typical emerald. The largest crystal, which had been found previous to 1891, measured 1 1/4 inches in length, and the largest faceted stone weighed 2 1/8 carats.

A number of fine emeralds have been found in North America. Small crystals have been met with at numerous places in the eastern parts of the United States. In the State of North Carolina they occur in druses in gneiss at many places in Alexander County, and especially at Stony Point. Here also are to be found other varieties of precious beryl, together with hiddenite, the so-called "lithia-emerald". During the course of a few years stones to the value of 15,000 dollars ($334,600) were obtained at this spot by the Emerald and Hiddenite Mining Company; at the present time, however, the mine appears to be exhausted. Only a few of the emeralds found here were suitable for cutting as gems; the largest and finest stone found yielded a faceted gem weighing 6 carats. Russell Gap Road in the same county is also mentioned as a place where emeralds have been found. A few good crystals have been met with near Haddam in Connecticut, and near Topsham in Maine, but the occurrence of emerald as a whole in North America has only local significance.

In former times Brazil was considered to be a country in which fine emeralds abounded, and after the Portuguese conquest strenuous efforts were made to discover naturally occurring stones. Not a single emerald, however, has been found in this country, and it seems probable that green tourmaline, which abounds in Brazil, was mistaken for emerald. It is certain that in ancient times the name emerald was applied loosely to a large number of green stones, such, for example, as green jasper, chrysocolla, malachite, and others. Even at the present day the name with a distinguishing prefix is applied to several green stones; thus "oriental emerald" is green corundum; "lithia-emerald" is hiddenite, a green mineral belonging to the pyroxene group and found with the true emerald in North Carolina; "emerald-copper" is dioptase, a beautiful green silicate of copper. The two latter are both used as precious stones.

The green minerals which are sometimes substituted for the emerald, and which may be mistaken for it, include green corundum, known as "oriental emerald," green garnet, known as demantoid, hiddenite, diopside, alexandrite, green tourmaline, and perhaps also chrysolite and dioptase. Each of these minerals has a higher specific gravity than the emerald; each sinks in liquid No.3, and some even in the heaviest liquid, while the emerald floats in both. Moreover, the "oriental emerald" is much harder; the demantoid, the colour of which has usually a yellowish tinge though sometimes very similar to that of the emerald, is singly refracting. Hiddenite is very rare, and is considered to be more valuable than the emerald; it is used as a gem practically only in America. Diopside is much more of a bottle-green colour than is the emerald. Alexandrite is distinguished from emerald by its hardness and its remarkable dichroïsm. The colour of green tourmaline, though often not dissimilar to that of pale emeralds, is frequently distinctly bluish in character; this mineral is easily distinguished from emerald, however, by its specific gravity, which is 3.07 g/cm3, slightly greater than that of liquid No.3, in which, therefore, it sinks. Chrysolite is yellowish-green, and can be distinguished from emerald by its colour and its faint dichroïsm. Finally dioptase is always of a very dark emerald-green colour; it is only semi-transparent and far softer than the emerald.

A glass of a fine emerald-green colour may be obtained by fusing together 4608 parts of strass, 42 parts of pure copper oxide (CuO), and 2 parts of chromic oxide. It differs from emerald in being optically isotropic, in the absence of any trace of dichroïsm, and in being much softer. An imitation of emerald, which contains from 7 to 8 per cent of beryllia is sometimes put on the market at the present time. It has a fine emerald-green colour, but is not perfectly transparent; it encloses numerous small air-bubbles and is not dichroïc; its specific gravity is 3.19 g/cm3. It is obvious that we have here a glass to which beryllia has been added in order to give it a chemical composition similar to that of true emerald.

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This document is in the public domain.

March, 2011