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|  | | | |  | Synthetic Corundum | | One common misconception with synthetic corundum is that it is made from a different substance to natural corundum. Synthetic corundum is in fact is made from exactly the same material - aluminium oxide (Al2O3). Synthetic corundum was first produced commercially in the form of ruby around the mid 1880's. Today huge amounts of synthetic corundum is produced along with many other types of synthetic gems, including diamond. The difference between synthetic and natural corundum is the way they formed. Natural corundum has been produced by nature whereas synthetic corundum is a man-made substance produced in a lab/factory. Synthetic corundum has many industrial uses due to it's physical properties. Corundum is both very tough and second only to diamond in hardness. Both types of corundum have the same physical properties and will often produce similar results when tested. It is therefore often down to careful examination of the inclusions within the stone to separate natural and synthetic corundum. Below is a brief description of the types of synthetic corundum commonly encountered in the gem markets. | 
| The first type of synthetic corundum is known as Verneuil or Flame Fusion. The process was invented by August Verneuil in the late 1880's. The process used to produce this corundum is relatively simple and inexpensive and is still used today to produce a variety of synthetic gems. The method is quite simple. A powdered mixture of the ingredients is slowly dribbled trough a flame. The molten mixture | | then collects on a rod below, which is slowly lowered. The molten mixture cools and solidifies forming a crystal or boule. These boules are then split down the centre to reduce the stress and prevent them fracturing. This type can be relatively easy to separate from natural corundum as the stones often show curved striae (curved colour banding) and gas bubbles. These inclusions are never found in natural rubies & sapphires. Sometimes these stones are heated and quenched in a liquid. This can give the stones inclusions which mimic natural fingerprints. The fingerprints often form a distinct pattern and curved striae is often still visible. |
| | The second type of synthetic corundums are known as Flux Grown synthetics. This type of synthetic can be far more difficult to separate form natural rubies and sapphires, as the flux inclusions can often imitate those found in natural corundum. There are a variety of methods used to produce flux grown synthetics but the resulting gems are very similar in appearance. Corundum has a very high melting point (2044±4°C) so fluxes are used to lower the melting point of the mixture. During the crystallisation process the crystals often form stress fractures when pressure is released during the manufacturing process. The fractures are then filled with the flux growth solutions, which heal the fractures but leave behind a pocket of undigested flux. This flux then forms an inclusion, which can strongly resemble feather or fingerprint inclusions found in natural corundum. However, an experienced eye should be able to separate synthetic flux stones from natural gems by closely examining the flux inclusions under a microscope. The difference in the refractive index of the flux to the corundum, will make the flux stand out in high relief or make them highly visible. The flux inclusions also appear more course in comparison to the graceful appearance of natural feathers or fingerprints. Sometimes, small blobs of flux resembling drops of water can also be seen. Another inclusion sometimes seen in flux grown stones, are small black specks of platinum which appear to have come from the crucible, which the crystals were grown in. However, not all flux stones contain these dark specks or platelets. One must also take into account there are many different processes used in the production of flux grown rubies & sapphires, each having its own distinctive trademark inclusions. |
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| | | Doublets | Doublets are basically a compound of two different materials glued together to resemble a natural faceted or cabochon cut gem. They can be found resembling variety of gems from opals, to emeralds, rubies and sapphires. Ruby & sapphire doublets used to be fairly common in the 80's and 90's but I rarely see them now. That is not to say they have disappeared - they still sometimes appear mixed into parcels or on their own. Most doublets are produced by using a synthetic or an imitation backing, glued to a natural crown. This is the most common type of ruby or sapphire doublet found. The natural material is often inexpensive green or white sapphire glued to a synthetic backing. When viewed through the table it is often possible to see the glue where the two pieces have been joined together. It is also common for the crown to show natural inclusions like feathers or crystals. If the stone is viewed with a 10x loupe from the side it is possible to see where the pieces are joined. The join can also be clearly visible if the stone is immersed in a liquid with a similar refractive index to the stone being viewed. Opal doublets are assembled using a thin layer of crystal opal glued to a black backing, often black potch. Black potch is black coloured opal that does not display any play of colour. |
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