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Heating has a long history among the possible methods of gemstone enhancement on a variety of gemstones. Corundum is the most representative example. Almost all rubies and sapphires have been heated at higher temperatures than other gemstones and many are heated more than once until the desired colour change is achieved.
For this reason, some of the effects of heated can be observed even through normal gemstone testing. Repeated heating at high temperature effects the internal texture of corundum: the inclusions and growth texture may be peculiar. Fluid inclusions can become wavy in appearance, similar to the flux feather-type inclusions in synthetic stones, instead of flat as they appear in unheated stones. This is very common in heated ruby. Fluid inclusions may also become film like.
The texture of silk also changes. There may be tension cracks around crystals.
Fluorescence under ultraviolet light is a feature of some heated sapphires. Almost all heated geuda emit chalky blue fluorescence under short-wave UV.
But in these cases, the determination of whether or not the corundum has been heated is subjective on the tester's part and the conclusion is not necessarily correct.
As you know, in Japan almost all jewellery is sold with gem testing reports which act like written guarantees for the purchase. In Japan we have a system created by the Japan Jewellery Association and the Association of Gemmological Laboratories, whose 30 members issue 90 percent of the certificates in Japan, for disclosing gemstone enhancement on certificates which went into effect on June 1, 1994.
According to the rule book, which is called The Definition and Nomenclature for Gemstones and Materials used in Jewellery, in addition to mentioning the natural origin of the corundum, any enhancement or treatment must be mentioned in the comment section.
Because heating is common in corundum and most testing is subjective, for corundum the comment "Natural ruby is generally enhanced" or natural sapphire is generally enhanced" is added.
In Japan, it is only permissible to issue a report without the general comment about enhancement if it can be certified that it has not been heated by scientific methods rather than subjective tests.
One possible scientific test which can be useful for judging if a corundum has been heated is spectroscopy. As the condition of trace elements is changed by heating, the spectrum of the stone may show a difference before and after heating. In particular this effect can be seen in blue sapphires from a particular country.
But the most powerful scientific method for determining if a ruby or sapphire has been heated is laser tomography, which is also known as light-scattering tomography.
What is laser tomography? It is a way to observe micron or sub-micron size features which cannot be seen by a normal microscope. But if a light beam strikes these small features, it scatters and it is possible to observe this Tyndall phenomenon. Laser tomography uses a narrow laser beam to scan the substance. A tomograph is then taken of the scattering effect produced. With this technique, micron size substances and imperfections in the crystal structure can be observed in transparent gemstones.
Laser tomography allows optical study of features which had to be examined in the past by x-ray diffraction topography. Because it is optical, laser tomography is comparative y easy to operate and does not require damaging the sample which makes it very useful for examining internal features in transparent gemstones.
There is no doubt that inclusions and the growth texture observed in crystals reflect the condition and history of crystal growth. These features have long been visually examined by gemologists for their colours, shapes, distribution and so on under low magnification using ordinary microscopes.
However, there are limits to this type of investigation because micron and sub-micron size impurities, distribution of lattice defects, or heterogeneous partitioning of impurities are difficult or impossible to visualise with conventional methods. These properties are important evidence of the growth process and are also the most affected by later heating at high temperature.
Laser tomography has many merits. First, it is possible to obtain even a very faint scattering figure from crystalline imperfections because the narrowness of the laser beam allows the elimination of stray light. It is possible to detect growth banding, growth sector boundaries and dislocations in addition to the presence of light-scattering sub stances of sub-micron size. However, because there can be light scattering from the surface of faceted stones, it is advisable to observe a sample immersed in a
In addition, when a tomograph is taken, the laser beam scans slowly through the sample on the same level. It is possible to take a scattering figure of any level section by changing the orientation of the beam. Similarly, a level can be scanned in any direction. This enables the tester to obtain a three dimensional image of the distribution of crystalline imperfections in the stone.
Because the laser beam is very strong as well as very narrow, feeble scattering figures can be detected using a photographic camera even if they can not be seen with an optical microscope.
The most effective laser for gem testing is an argon ion laser (blue at 488nm) because it not only reveals a clear scattering figure, it also reveals a fluorescence figure, since the argon laser excites fluorescent elements and gives an image that shows the distribution of fluorescent elements. This allows correlation between spatial distributions of crystalline defects and impurity elements.
It is a great advantage for gem testing to be able to take a light scattering tomograph and a fluorescent tomograph at the same time because many fluorescent elements like chromium play an important role in major gemstones.
Because the heating of corundum involves high heat, often with repeated exposures, heat distortion of the crystalline structure and changes in state of the crystalline impurities are especially apparent.
The most important distinction between heated and non-heated corundum is seen in the distribution of microscopic substances which is not visible by ordinary microscope but can be seen with laser tomography. If the substances are condensed into a certain area with sharp boundaries, this is evidence that a stone has been heated.
Generally scattering pat terns are much clearer in sapphire than in ruby. In particular a scattering pattern of triangles can be taken as sufficient evidence that a sapphire has been heated.
In contrast in non-heated sapphires and rubies needle shaped substances can be seen distributed in three directions.
A fluorescence figure which is partially orange with a background of red can be seen in heated rubies. I have so far not seen a tomograph like this among non-heated rubies. In sapphire, sometimes a fluorescence figure with different intensities can also be seen, also indicating heating.
Lattice imperfections can also be seen. Bundles of radial dislocations, radially developed from a plane toward the outside of the crystal can often be seen in heated rubies. These dislocations are due to the stress of thermal gradients.
In the future, the observation of crystalline imperfections and defects like dislocations and growth banding should become indispensable in gem testing. The use of the laser tomograph to obtain this data will become increasingly important.