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| Gemmological microscopes with low magnifying power (commonly from about 10X to 60X) are used for standard gemmological tests. Needless to say, the instrument is used to inspect inside of a gemstone. Observation of inclusions or growth structure will make you possible to reveal origin of the stone because environment and history in which a gem crystal grew are reflected into those features. One of effective means used as laboratory techniques to observe defects inside a crystal is X-ray diffraction topography, which is mainly applied to inspect semiconductor materials or minerals. However, this method requires sophisticated technique all the way from its operation through the analysis of the image obtained, moreover, the size of samples is very restricted. Therefore, this method is not appropriate to be used in the field of gemstone identification. Contrary to this, laser tomography that makes similar observation possible as a microscopy is rather easy to operate and there is no need to concern about any damages to the gem samples. ◆Laser tomography Minute substances, which cannot be directly observed their true figure by an optical microscope, can be observed by utilising Tyndall phenomenon that is produced when beam of light illuminates the substance. This observation method using light scattering has long history. The existence of minute scattering whose size is beyond the resolution limit of an optical microscope was confirmed in 1903 by using an ultramicroscope. A research group lead by Dr. Kazuo MORIYA et al. at Gakusyuin University developed in late 1970's a method to obtain three-dimensional tomography using narrowed laser beam called “light scattering tomography" that is scanned in a sample. This method has superior advantages shown below, which are quite suitable for observation of nonuniformity in a transparent crystal. GAAJ was among the first to apply the light scattering tomography to gemstone identification, and with the assistance of Dr. MORIYA, the technique was developed to a practical level in 1983. 1) As narrowed laser beam is used in this instrument, stray light is eliminated and thus faint scattering figure such as crystal defect can be seen as its original state. According to the study by Dr. MORIYA et al., growth stripes, growth sector boundary or dislocation (linear defect) other than light scattering in submicron size are detectable. However, when observing a gem with many facets that have been cut in various directions regardless of crystallographic direction, certain ingenuity in taking laser beam into a sample is required. To prevent reflection from the surface of these facets and to incident a laser beam effectively into a sample stone, the stone should be observed with being immersed in a liquid whose RI is as close as possible to that of the sample stone. For example, methylene iodide (whose RI is about 1.745 at room temperature) is suitable for corundum, but diamond (whose RI is 2.417) has no suitable immersion liquid. |
2) Tomograph of inside of a stone is taken while slowly scanning a laser beam at a certain level in a sample stone. By varying the depth of scanning beam in a sample, any selected section image can be obtained. Moreover, observing the sample in the same way with varying the direction of the sample enables you to comprehend nonuniformity of the crystal three-dimensionally. 3) Laser tomography not only can observe very weak scattering image but also can take its record photograph clearly. In this case, optical magnification is only dozens-fold, but as for a gemstone, the power as low as this is quite appropriate to observe overall structure of a stone. 4) Many types of wavelength can be used as an origin of laser, but argon ion laser (blue colour) with wavelength of 488nm is the most suitable for this tomography. Because, other than clear observation of scattering image such as crystal defect, laser tomography may also be able to observe fluorescent image excited by argon laser. In other words, observation of scattering tomograph and fluorescent tomograph can be obtained at the same time. Explaining in more detail, fluorescence is a phenomenon that an electron in a luminescence centre excites by receiving external energy such as light and discharges the energy (emits light) when it moves back toward the ground state. The wavelength of emitted light then is longer than the wavelength of the energy for the excitation. For this reason, blue light of shorter wavelength is useful to observe light emitting (fluorescence) in the range of visible light. Fluorescence colours of green, yellow, orange or red can be observed after excitation by blue light. Contrary to this, excitation by red light cannot result in emission of wavelength from blue to orange, and red fluorescence against red laser is quite difficult to observe.
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| ◆Observation examples of various gemstones Certain level of crystallographical knowledge and grater amount of experiences are vital to interpret scattering image or fluorescent figure obtained by laser tomography. To improve our knowledge, GAAJ research laboratory launched a study group and continued research activity. During the period over twenty years since we were equipped with the laser tomography, we could accumulate abundant data on each gem variety. With this background, we feel proud that we have established the method which is inevitable at present in detecting heated or non-heated origin of corundum. Now some observation samples of photographs taken in early 1980's when our study just began are shown in the photos. |
Photo-2a is an example of sector zoning seen in natural alexandrite. Natural crystals usually contain rather complicated domain structure like this. Contrary to this, shown in the photo-2b is a flux-grown (synthesised by Creative Crystal) synthetic alexandrite that was said to be extremely difficult to identify those days. It shows quite uniformed zonal structure, as it has been grown from a seed crystal in constant atmosphere. Photo-3a shows scattering image corresponding to Brewster fringe seen in natural amethyst. This structure results from Brazil law twinning, and this still can be regarded as a proof of natural origin today. Shown in the photo-3b is a synthetic amethyst produced in Japan those days. A seed crystal (parallel to the table facet) that is almost invisible under a gemmological microscope can be clearly seen here. Around 1980 was the period that had no definite method to distinguish synthetic amethyst from natural, but laser tomography was already in a practical use to detect structural difference between the two. |
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