“Kaleidoscope” in Alexandrite
Recently, the author examined an interesting 0.69 ct alexandrite measuring 6.22 × 5.21 × 2.51 mm. This stone displayed a color change from greenish blue in fluorescent light to purple in incandescent light. Its trace element chemistry along with the presence of characteristic inclusions suggested a Russian origin.
Under the microscope, this alexandrite showed a unique stellate structure that was divided into six sections of purple, pink, and yellow color. Chrysoberyl occasionally shows a stellate structure due to cyclic crystal twinning (trilling), with triplets oriented at 120° to one another (e.g., K. Schmetzer, Russian Alexandrites, Schweizerbart Science Publishers, Stuttgart, Germany, 2010). Each section showed a different face-up color, and the colors were changed by rotating the stone. This observation suggested that the stone was cut from a cyclic twin rough crystal and the various colors were due to pleochroism, not color zoning. When rotating a single polarizer between the stone and objective lens (analyzer), opposite areas showed the same pleochroic colors (see above). This feature indicated that the alexandrite consisted of three penetration twins (again, see Schmetzer, 2010). The unique pleochroic colors caused by cyclic twinning were reminiscent of a kaleidoscope.
