The emerald is a variety of beryl and is precisely the colour green.
The beryl in nature comes in relatively complete hexagonal prisms, sometimes with basal faces or small bipyramid faces. The sizes of the crystals can also be quite considerable: easily a few centimetres, up to a few decimetres; Some of them have been found, though sporadically, even more than a meter. Oftentimes it is turbid; When it is transparent it has a vitreous gloss. Most of the time it is whitish, opaque, lather-like, or just yellowish. or a very light grey, or just greenish. However, it may also present more decisive and attractive colours: mainly blue, green, yellow, pink. It exists, albeit rare, the red and uncommon variety is also the colourless variety. When bright colours are associated with some transparency, there can be gems among the most valuable including the emerald.
Physical properties: It has a hardness of 7 1/2 up to 8 associated with a certain fragility. It may have a splash hint according to the base. It has a density around 2.67-2.72 g / cm3, which can however sometimes also reach 2.90 g / cm3. The refractive indexes, such as density, are quite variable, from 1,565 – 1,570 to 1,590-1,598.
Origins: It is an accessory mineral of granite-type rocks; It crystallizes mainly in pegmatites, where larger crystalline individuals are found. It is also formed for metasomatism in the pegmatite encasing rocks. It is sometimes connected with high temperature hydrothermal conditions.
Emerald Variety: The emerald is therefore the variety of green coloured beryl. Not all of the green beryls of gem quality are called an emerald: when the colour is green-yellow, it is said as a matter of fact a heliodor; When the colour is light green-blue, but also small green (due to iron and not chrome as in the emerald), it is referred to as an aquamarine.
The name has ancient origins: the Latin smaragdus seems to indicate the stone we call the emerald.
The typical emerald colour is a particular green called precisely an emerald green; it is owed to the presence of small amounts of chromium in the crystalline structure. However, it can also be light green, or dark green, bright green or a leaf green colour. The gloss called vitrea is not exceptional among the gems is more noticeable in medium-light coloured stones and with very few inclusions. There are always present inclusions, which in the best quality emeralds are very soft, so that they are not visible to the naked eye while they are visible from the 10 enlargements. The most widely used cutting shape is the rectangular table and stairs shaped form which is called an emerald cut. It is also cut oval, or a drop, or cushioned, with a mixed cut, but not often, while old or ancient stones meet table cuts and hexagonal contour steps, cabochon cuts, or rounded in and drilled general terms, pear shaped mostly used as pendants.
The colour of emerald when it is typical, is hardly confused with other gems. The emerald can have very characteristic inclusions for those who have a bit of familiarity with this particular aspect of the buds: there may be liquid or two-phase residues with a gas bubble inside fusiform cavities or more rarely, prismatic to extreme trunks; There may be crystalline lamellas with rounded contours, birefringent from mica; There may be crystals of rich pyrite rich faces, or rhomboids of calcite. In order to recognize them however, the use of a microscope is almost necessary. Certain green tourmalines, though having a colour that is not typical of emeralds, they may resemble them, however if they distinguish some of them sometimes by the very marked pleocroism. The olivine or peridot may also have an intense green colour vaguely similar to that of certain non-typical emeralds: the strong birefringence of the olivine can however also be revealed with a simple magnifying glass, observing from the table in certain directions sharply doubled images of opposing edges. In any case, even density detection alone would allow the distinction to be made immediately.
The most popular emeralds come from Colombia, where the most famous mines are those from Chivor and Muzo. Also many light-coloured, pale-coloured stones come from Colombia’s fields. More limited emeralds come from Brazil. Small and very intense stones come from the Transvaal, features for the numerous minute inclusions of metallic molybdenite. Over recent decades, an increasing number of emeralds have come from a number of small fields located in eastern Africa Zimbabwe, Zambia and Tanzania primarily. Of fairly intense colour, sometimes with bluish green tones, often contain mica lamina and sometimes thin crystalline needles. The most famous are those in Sandawana, Zimbabwe, which are appreciated for their colour. Emeralds with similar features also come from the mountains of Pakistan and India, as well as from the Soviet Union (Ural Mountains).
Beautifully coloured stones and weighing more than 2 carats have valuations in the field of precious stones and can equal or exceed those of the diamond. However, the value decreases considerably even when the tint colour is not the typical type, or even when the colour is too dark or too light. When a certain turbidity is then added, the value decreases even more considerably.
It was established that the Romans already imitated the emerald with carefully carved green glass. Glass has also been used in centuries closer to us, sometimes incorporating foreign particles to simulate inclusions. They are also used as imitations, doubles with the bottom of green glass and the top of garnet, or triplets with a layer of coloured mastic between two coloured layers of beryl, synthetic spinel or quartz. For a few decades it has begun and it has taken on the development the production of synthetic emeralds, which are generally quite beautiful in colour, distinct from the natural ones, not so much for the physical characteristics, which are the same, but mainly for inclusions and other peculiarities of growth. These low-value synthetic stones are on the market by now in large quantities.
Chemical composition and colouration:
Chemically, the emerald is an aluminum and beryl bisilicate in which impurities are present. The colouration in the emeralds is due to chromium oxide in quantities up to 0.20% and detectable, spectroscopically, even in very little coloured gems. Also the spectroscopic analysis revealed the presence of vanadium in the intensely coloured gems. The presence of this element in natural emeralds was observed by the geochemist V. M. Goldschmidt since 1919. Furthermore examining emeralds coming from Norway, he found: 0.90% V2O3 and only 0.1% CrO3; In the emeralds of Muzo he found a significant presence of both elements. Recent analysis on the Emerald of Chivor gave: 0.05% V, 12% Fe and 0.14% Cr. In Brazilian emeralds a remarkable amount of vanadium was found: chrome 0.003% and vanadium 0.15%. The presence of vanadium in emeralds raises the problem of establishing a demarcation line between green and emerald beryl. This is based on two airy proposed definitions: “the emerald is a green variety of beryl,” or “the emerald is a green beryl chrome coloured variety? » The most accepted and undoubtedly the most logical remains the first since the second would raise the question of how much chrome is necessary both present to define emerald a green beryl, then leaving the presence of vanadium and iron unsolved! The most commonly accepted definition is therefore “The emerald is the green coloured beryl” regardless of the element (or the elements) upon which colouration is necessary. As far as Colombian emeralds are concerned, through repeated analysis carried out on Muzo’s emeralds, the presence of chromium and iron oxides has been found in the following proportions:
Fe203 1,005% CrO3 0,264%
Emerald colouring has a wide range of intensity for rubies and sapphires: From light green it comes to an intense bright green colour and often hollow. In the emerald, the greater or less intensity of the tint is not in line with the field, as it has been seen that both in Colombian and Russian mines can they can reach of fine coloured green gems, such as others of light, faded colour. Those of Brazil are generally very tending to the yellow types, but the emeralds of Salininha and Carnaiba have a good green colour. Whether or not the emergence of emeralds is very difficult, more than it is for rubies and sapphires. Commercially however, this assessment should not be of great importance since the value of emerald is predominantly given by its colour without prejudice to its origin.
In the Colombian emeralds the density is generally corresponding 2.67 and a little higher to 2.69-2.70, in the Urals. For those from Brazil there are values around 2.67-2.72.
The refractive indexes entail more or less such as the density and that is, 1,581 / 1,575 for the emeralds of Colombia;
For the Ural emeralds 1,584 / 1,578. For those from Brazil, the two indices are slightly higher and even more so than the South African ones.
Data on refractive indexes for Sandawana emeralds are among the following: 1,5890-1,5943 / 1,5830-1,5874
For those from Pakistan 1,595-1,600 / 1,588-1,593;
Summary index table of refraction / birefringence indexes and density based on origin
Colombia (Muzo) . . . . 1,569-1,584 1,563-1,578 -0,006 2,69-2,7
Brasile . . . . 1,571-1,573 1,565-1,568 -0,005/0,006 2,67-2,72
Colombia (Chivor). . . . 1,575-1,581 1,569-1,575 -0,006 2,688-2,70
Russia (Urali). . . . 1,581-1,588 1,574-1,581 -0,007 2,70-2,74
Brasile. . . . 1,586-r,591 1,580-1,583 -0,000/0,008 2,72-2,75
Transvaal (Leysdorp) . . . . 1,587o6-1,593 1,58o50-1,585 -0,00656/0,008 2,718-2,78
Zambia . . . . 1,588-1,590 1,58o-1,58 -0,008/0,999 2,71-2,76
Rhodesia (Sandawana). . . . 1,5890-1,5943 1,5821-1,5872 -0,0069/0,0071 2,74-2,76
Austria (Habachtal) . . . . 1,5907-1,591 1,5839-1,584 -0,0068/0,007 2,72-2,76
Norvegia (Eidsvold). . . . 1,5908-1,591 1,5838-1,584 -0,007o 2,74-2,759
Indi. . . . 1,591-1,595 1,584-1,588 -0,007 2,72-2,74
Pakistan. . . . 1,595-1,600 1,588-1,593 -0,007 2,75-2,78
In the emerald the dispersing power is rather low.
It is not susceptible, but more noticeable with increasing the intensity of the tint
Hardness: – The hardness is between 7.5 and 8 and it can be said that in individuals where the density is among the highest values, as well as the refractive index, the hardness is slightly higher than the others. For the Colombian types it may also be slightly lower: 7.25-7.5. In the emerald the cleaning is still very good, it is less durable than in other stones, so it needs to be renewed more often than other gems of the same hardness.
Heating action and luminescence effects at various radiations. – The heating effect has no influence on emeralds, although some creators admit that operating between 300 and 600 °C, the transparent green emerald becomes opaque. The luminescence effects are almost zero to the X-ray, while cathode ray is noticeably a red fluorescence more or less darker; Ultraviolet rays have different behaviours depending on the wavelength. In fact, long UV wavelength emeralds from Colombia easily present a red-pink fluorescence, sometimes a little stained and no fluorescence for UV rays. with a short wavelength. In general, all other sources do not produce UV fluorescent rays.
The cutting. — Usually the emerald is cut to square or rectangular shapes with a large central table at the top (crown); In the lower part (pavilion) and it is very important that the cut is approximated to the shape of a parabola, for the purpose that the gem becomes illuminated in all its measure, through the light rays that are refracted from the cone. At the bottom part, the step style cutting is applied; Sometimes however emeralds are differently faceted depending on the different colour intensity. Given the preciousness of the material however, it is sought out to lose as little as possible on the weight of the raw, exploiting the best colour.