Synthetic Spinel Gemstones - Great synthetic stones
It is quite by accident that the synthetic spinel was produced in the mid of the 18th century; the phenomenon has been the subject of various scientific publications written between the years 2000 and 2004. When scientists, in the early part of the 20th century, tried to manufacture synthetic blue sapphire, they accidentally stumbled upon synthetic blue spinel. It is from this time that this synthetic form of spinel was used to substitute the natural counterpart. In the 1990s, synthetic spinel having various colors such as red, was produced in Russia using the flux-grown method; red could never have been produced using the flame-fusion process.
It was quite by accident that the first attempt to make a good quality blue synthetic sapphire, using the Verneuil process, yielded the first synthetic spinel. The scientists assume logically that the addition of cobalt oxide to the alumina would end up getting a synthetic blue sapphire, but this was not the case, since the resultant crystal had an uneven distribution of color. After this, they thought that by adding magnesium oxide as a flux, they could make the cobalt oxide color to be evenly distributed in the resultant crystal. Well into making the color even, they hit the jackpot, but upon examining the crystal, they saw significant differences from natural sapphire. They found that the boules of the crystal had a square outline and that the resultant material was not that of a sapphire but that of a magnesium aluminate that resembled a spinel and not a corundum.
Most other synthetic gemstones have a composition that is very similar to that of their natural counterparts. However when it comes to synthetic spinel, there is a lower content of magnesium oxide. The natural spinel has a ratio of Al2O3 and MgO of 1:1. But the synthetic spinel has a ratio of 2.5:1. It is this extra alumina which causes the uncharacteristic double refraction which is seen when using a polariscope. It is the straining of the crystal lattice by the excess alumina, which changes the optical and physical properties of synthetic spinel. There is no way that one can make spinel, which resembles the natural form using the Verneuil Process. With this method, you do not get a boule of one crystal only, but you get a number of crystals with different orientations, forming each boule. As soon as such a boule cools down, it will certainly crack.
The early investigations of this material brought about the conclusion that the extra alumina was retained in its cubic form, which is called gamma corundum. Therefore, the synthetic spinel which is commercially produced is actually a mixture gamma alumina and magnesium aluminate. The huge difference in the composition of the synthetic spinel when compared with the natural one is the reason why the accuracy of the term spinel, when used to describe the synthetic variety is questionable.
Apart from the color Purple, you can find synthetic spinel in all other colors; it is also possible to find the deep red colors that are associated with rubies and garnets. Since the formation of the ruby red synthetic spinel, there has been hope that they will soon be available for commercial exploitation and available to the markets. The ruby red varieties were produced from a solution. A platinum plate was used to grow the crystal, which had a higher specific gravity and very high refractive index. The X-ray diffraction was similar, in pattern, to that attributed to natural spinel.
There was only one occurrence of a similar stone, which was in the form of a cut specimen, and was found in Paris. What was not so clear was whether the stones were a result of someone having found a way to make synthetic spinel, which could easily be confused for the natural thing, and keeping it as a secret, or whether they were just a part of a hydrothermal lab experiment.
Later on, in Lucerne, Switzerland, Dr. Edward Gubelin, was able to describe ruby red spinels, which were very different from any other spinel that had ever been produced in the world. These fine specimens had a specific gravity and an index that was so close to the natural red spinel, that one could easily be duped. These spinels did not have the high alumina content that was found in spinels made using the Verneuil process. They were examined and compared to caliber-cut red spinels from Brazil, in their small sizes of less than o.25 of a carat and found to be astonishingly similar to the natural spinels.
The saga of the development of synthetic spinel went on, and in 1960, it was possible to get boules that weighted about 200 carats or 400 grams, and you could get 2 and 5.5 carat stones. The material was of high quality and had a refractive index ranging from 1.722 to 1.728; they all had a specific gravity of 3.6. It was possible to see plain curved striae, when the stone were magnified for immersion; it was also possible to see large irregularly elongated bubbles, and other small spherical ones.
There were also flat tiny cavities that were two-phased in structure and each had a gas or liquid and they were almost always joined by a tube. The stones showed remarkable phosphorescence when they were exposed to X-rays, and had a blood-red color, when they were exposed to long-wave and short-wave ultraviolet rays.
In 1954, in Idar-Oberstein, Blue stones, which were very similar to lapis lazuli were synthesized. They were classified as cobalt-colored spinel, and they were made using a completely new process. It was found that the resultant crystal was composed of many small crystal grains and was not a single crystal on its own. When it was introduced, it was said to be a sintered synthetic spinel that had the color of lapis lazuli. This was the result of finely ground spinel being sintered and then recomputed using pressure; sintered means that it is semi-fused at high temperatures of 2135 degrees centigrade. One aspect that is notable is the fact that the pyrite inclusion seen in lapis lazuli can be imitated using gold filings. In the case of the synthetic stone, fool´s gold is taken as genuine, and the real gold in the real stone is thought to be an imitation; talk about significant irony.
This is a material that is very desirable when it comes to making gems, because of the fantastic colors and clarity of the stones. The resultant gem has a hardness if 8 on the Mohs scale, making it excellent for faceting. The rough is made in plates which are about 2-3 millimeters thick, or as spheres with several millimeter diameters. Imitation lapis lazuli is now being produced in a new form by Pierre Gilson S.A. Lapidatres, and it is now the most preferred substitute for the sintered synthetic blue spinel.
It is possible to get another form of spinel when colorless material is heated for a long time, such that the alumina separates into small corundum crystals. This separation causes the stone to have a cloudiness that brings about a wonderful and realistic effect that makes it a great moonstone imitation. This is a gemstone that has an orientation that can bring about a star effect, which is made more apparent when a thin, almost transparent metallic layer is put at the bottom of the cabochon.
There are a number of reason why synthetic spinel continues to be produced. Firstly, it is slightly more malleable than synthetic corundum, and can quickly be fashioned into a gemstone and without much expense in terms of cost. Secondly, it is able to imitate the true colors of several natural gemstones better than synthetic corundum.
There is a huge difference between the boiling and melting point of synthetic spinel, as compared to that of synthetic corundum; this means that the manufacturers of synthetic spinel do not have to agonize over the process, so that no bubbles are formed; when making synthetic corundum, the process is delicate. Due to this difference, it is very rare to find bubbles forming in synthetic spinel, and when they do, they are usually very small and have an even distribution in the stone. It is also possible to find elongated bubbles that have an irregular make-up; other bubbles may also seem to be angular because they have many flat faces. Synthetic spinel is so perfect that you can hardly find the kind of curved striae that you find in synthetic corundum, that other methods are used in order to identify it properly.
One of the methods that are used to identify the synthetic spinel is combining the refractive index which is much higher than that of the natural spinel together with the double refraction which has a patchy pattern, which can be seen using the polariscope.
Another method is to use the color filter. This was a method that was used to separate the glass imitations from the natural emerald stones. When synthetic emeralds and other plastic imitations are put under the filter, they appear to be red in color. The filter has been used in telling the distinction between natural and synthetic spinel, that the name spinel filter is now being proposed for it. For example, synthetic blue spinel is made to imitate aquamarine, but when it is viewed in the filter, it will appear blue; the natural aquamarine will maintain its natural color of blue. This is the same case when telling the difference between synthetic blue spinel that is used to imitate blue sapphire.
In some other cases there needs to be a more detailed testing of synthetic spinel in order to separate it from the natural gems that it is imitating. The cobalt that is used to give the synthetic spinel a blue color can be seen when a spectroscope is used. The blue lines are easily seen at 6359 A.U. in orange and at 5800 A.U. in the yellow. It is also visible at 5400 A.U. in the green.
The band in the yellow and the orange are broad, and are usually about twice as thick as those found in the green, when it comes to blue, and lapis colored synthetic spinel. When observing light blue material, the bands of all three colors are considerably weaker.
It is also possible to see a bright red luminescent color, which is visible as a line, when viewed in the deep red. Due to the natural ferrous metal found in natural blue spinel, the band shows at 6320 A.U. in the orange. The band is thicker and stronger when viewed at 4800 A.U. and 4590 A.U. There are two lines that are seen when Yellow Spinel, the synthetic variety, is seen in the bleu violet at 4450 A.U., and in the violet at 4220 A.U. Any material that has a Greenish Blue color will show a spectrum that is a combination in the blue of cobalt and yellow of manganese. Stones similar to alexandrite will absorb a lot when viewed in the yellow green.
It is not difficult to detect sintered spinel that has a lapis lazuli color. Usually the color is string, and it may seem reddish to those who are not familiar with these stones. The refractive index test gives a higher value for the synthetic variety when compared to the natural one. Synthetic lapis will appear red when put under the color filter as opposed to the natural one, which will not change at all.
These are the methods that help dealers to tell the difference between natural gems and the imitation made using synthetic spinel. There are many other methods that are used for this verification, but these are the most common. Sometimes an experienced jeweler or dealer can tell the difference, but so good are the synthetic spinel imitations, that they can easily be passed on as the natural forms. When you are buying gemstones, you should do so from a respected outlet.
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