corundum, inclusions.5, photomicrography, gems

A gemologist’s life would be far easier if gems were cut as parallel-sided plates. But they’re not. Facets are designed to reflect light back to the viewer, not transmit it. This means light entering the gemstone from behind (transmitted light) will typically not pass straight through. If we want to see inclusions, we must constantly change the light paths through the gemstone. This is done by changing the position of the gemstone relative to the light and changing the light relative to the gem. The most versatile and controllable method of doing this is with the fiber-optic light, an illumination technique “darker” than even darkfield. During a recent routine identification of a natural chrysoberyl a startling difference in two distinctly different types of illumination commonly used in the examination of gemstones was clearly revealed. These two methods are darkfield and fiber-optic illumination. When this chrysoberyl was studied in darkfield (Figure 1), only a fine linear growth band was seen. However, when fiber-optic lighting was directed in along the girdle plane, bundles of fine light-scattering silky needles extending at 90˚ from the linear growth band became abundantly clear. At the same time the linear growth band itself was also more clearly detailed (Figure 2). The difference in the amount of visual information provided by these two different lighting techniques was startling. In this instance darkfield was clearly inferior to fiber-optic illumination.