Monday, 8 July 2013

Synthetic Beryl

Synthetic BerylSilicate of beryllium and aluminium. Because of the value of emerald, efforts were made to synthesize beryl after corundum and spinet. The task proved far from easy, and mass production was only achieved around the 1950s. The basic procedures used have already been described. They are typically slow: it may take several weeks to obtain crystals of an acceptable size. They could be made to grow more rapidly, but the resulting crystals would be fractured, with numerous, highly characteristic inclusions, obviously making them less transparent and much more easy to distinguish from natural emerald. Synthetic beryl is normally only produced in the emerald green variety: colorless, pink, pale blue, and yellow crystals have occasionally been produced for research purposes, but they are not available on the market.


Crystal system

Hexagonal


Appearance

This depends on the method used. The flux melt process produces isolated, hexagonal, prismatic crystals or groups of prismatic crystals. Crystals have been obtained weighing 1000 carats or more, but growth usually stops much sooner. The prismatic crystals are very hard to distinguish, at first sight, from natural ones. The hydrothermal method produces crystals of more complex appearance, because to hasten their growth a bipyramidal seed crystal is used. Al one time, this second process was only used to deposit a thin layer of synthetic emerald (a few tenths of a millimeter) onto a seed consisting of a colorless, or almost colorless, piece of natural beryl (a variety of very low value) which had already been given a step cut. The stone was then carefully polished to give it the necessary luster with-out removing the coating, which was entirely responsible for the color. In both the flux and hydrothermal processes chrome is used to reproduce the green of natural emeralds.

Physical properties

Synthetic ringThese are much the same as for natural beryl. Nearly all synthetic emeralds produced by the flux melt process have slightly lower refractive indices and birefringence (0.003 or 0.004) than natural beryl. The density is also a bit lower, ranging from 2.64 to 2.66 g/cm3. However, the physical properties of synthetic emeralds produced by the hydrothermal process generally match those of natural emeralds perfectly particularly the comparatively rare types with a thin, synthetic coating.


Production

Having started up independently in Ger-many, the United States, and later Austria, this is now concentrated in the United States and France, but has apparently begun in Japan and the Soviet Union as well.


Synthetic emerald

The (emerald) green variety is the only one which is manufactured synthetically on an industrial scale. Production has been attempted by a number of companies, some of which have subsequently abandoned the task as uneconomical.


Appearance

It is usually quite a strong emerald green, with a bluish tinge, and, as with natural emeralds, the step or trap cut is normally used, mixed oval or pear-shaped cuts being rarer. Cut stones are generally quite limpid, al-though stones of this quality take longer to grow. All in all, their outward appearance is very similar, it not identical, to that of their natural counterparts.


Distinctive features

Synthetic emerald It was mentioned in the discussion on physical properties that accurate measurement could reveal differences between the refractive indices, birefringence, and density of natural emeralds and the synthetic ones produced by the flux melt process. The stones are normally distinguished, however, by their inclusions. In the case of hydrothermal synthetic emeralds, these are not very different from some inclusions in natural emeralds, but they are "oddly" arranged in relation to the growth lines and are also positioned like mirror images of one another on either side of a thin central plate constituting the seed onto which the synthetic product was grown. Emeralds produced by the flux melt process contain variable quantities of swirling, veillike inclusions consisting of whitish residues of flux material trapped in cracks that developed during their formation. But it is admittedly not easy to distinguish between synthetic and natural emeralds without a good microscope, a lens alone often proving inadequate.

Cost

The cost of stones with few inclusions is quite high, equal to certain prized secondary gems, but much lower than that of natural emeralds. Stones with copious inclusions are worth somewhat less, but are still among the highest-priced synthetic stones.

Writer – Curzio Cipriani and Alessandro Borelli

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