gems, synthetic chrysoberyl, Russia, horizontally oriented crystallization

Mineralogical and gemmological properties of synthetic alexandrites grown by the horizontally oriented crystallization (HOC) method are described. The method is used to grow crystal plates weighing up to 450 g. The morphology of the rough crystals is dependent on the form of the molybdenum boat used as a container for crystal growth. The most significant inclusions for gem identification are curved growth striations and flat, irregularly- shaped somewhat elongated cavities. Microprobe analyses revealed distinct amounts of chromium and vanadium but extremely low iron contents. Spectra in the UV-Vis range are consistent with published work. Six differently sized cubes were cut from an HOC alexandrite crystal and used to show pleochroism in combination with two optical phenomena: the alexandrite effect and the Usambara effect. Colorimetric parameters were calculated from the spectra of the cubes and plotted in the CIELAB colour space to demonstrate the alexandrite and Usambara effects graphically for different directions of view. The absorption bands in the infrared are polarized and assigned to structural OH-groups.