Challenges in Orienting Alexandrite: The Usambara and Other Optical Effects in Synthetic HOC-Grown Russian Alexandrite
These crystals, which range up to 450 g, are used both for laser applications and gem-quality synthetics. Since 1994, they have been produced for the gem market in a joint venture with the Tairus company in Novosibirsk. Among their distinguishing inclusions are curved growth striae (similar to Czochralski-grown material and sometimes accompanied by bubbles), elongated and irregularly shaped flat cavities (figure 1), some rare occurrences of tiny birefringent crystals, and the occasional appearance of irregularly oriented fibers and fissures. Of note is the very uniform distribution of the chromophores Cr3+ and V3+ within the material. The amount of Fe3+ found in this material is extremely low.
Of particular interest in this latest study of synthetic alexandrite, made possible by rare access to uniformly doped material, was the investigation of color related to various optical effects: the alexandrite effect (color change between daylight and incandescent light), the Usambara effect (variable coloration according to the path length of light), and pleochroism (coloration in views parallel to the three crystallographic axes). To accomplish this, the authors used a series of crystallographically oriented cubes with edges ranging from 2 to 10 mm. A particularly notable finding is that, similar to chromium-containing tourmaline, larger crystals display more redness (and less greenness) in all three directions of view. The Usambara effect, the variation of color with material thickness (assuming uniform lighting conditions), was first reported in 1997 (A. Halvorsen and B.B. Jensen, “A new colour-change effect,” Journal of Gemmology, Vol. 25, No. 5, pp. 325–330) and further described by Kurt Nassau in 2001 (The Physics and Chemistry of Color: The Fifteen Causes of Color, 2nd ed., Wiley, New York, pp. 94–96). Essentially, a material can have a variable transmittance in different areas, depending on the thickness of the material.
Additional color variations are described for the individual crystal orientations between daylight and incandescent light (figure 2). The subsequent discussion shows why it is difficult to ideally orient the table facet of an alexandrite with variable chromium concentrations and thicknesses. Also, there truly is no single orientation that maximizes the purest green and displays the most dramatic color change from daylight to incandescent light. This presents a formidable challenge to the gem cutter. Further research to investigate the impact of chromophore concentration (particularly Cr3+) on alexandrite’s color properties would be useful in fully describing this unique gem’s unusual range of appearances.