Abstract Gems & Gemology, Spring 2014, Vol. 50, No. 1

Challenges in Orienting Alexandrite: The Usambara and Other Optical Effects in Synthetic HOC-Grown Russian Alexandrite

Jennifer Stone-Sundberg

HOC synthetic alexandrite inclusion.
K. Schmetzer, H.-J. Bernhardt, W. A. Balmer, and T. Hainschwang. “Synthetic alexandrites grown by the HOC method in Russia: Internal features related to the growth technique and colorimetric investigation,” Journal of Gemmology, Vol. 33, No. 5–6, 2013, pp. 113–129. Photo courtesy of Karl Schmetzer.

Dr. Schmetzer and his coauthors contribute another gemological description of a unique source of synthetic alexandrite, the chromium-bearing variety of chrysoberyl. These crystals, grown in Russia since the late 1980s and similar in color to natural alexandrites from the Hematita deposit in Minas Gerais, Brazil, were formed using a direct melt gradient technique. This alexandrite crystal growth process, developed by Dr. Vladimir Gurov, uses the horizontally oriented crystallization (HOC) method. The crystals have all been grown along the b-axis in molybdenum crucibles that contain both the melt and the solidified portion.

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.

Optical effects in synthetic alexandrite.
Figure 1. The illustration demonstrates the three optical effects studied in the article: Pleochroism, the alexandrite effect, and the Usambara effect. Represented are two oriented cubes of HOC synthetic alexandrite with edge lengths of 3mm and 8mm viewed in unpolarized daylight and incandescent light down the a-, b- and c-axes. With identical lighting and edge length, pleochroism is clearly seen. The alexandrite effect is illustrated by comparing same size cubes along the same axis but under different lighting. Comparing the two different cubes under the same lighting and orientation, the Usambara effect is observed. Photos and artwork by Karl Schmetzer.
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.

Jennifer Stone-Sundberg is managing director at Crystal Solutions in Portland, Oregon, which specializes in crystal growth and characterization technology. A technical editor of Gems & Gemology, she holds a PhD in inorganic chemistry from Oregon State University.