Colourless Chrysoberyl from Mogok

first published in Facette 22 (February 2016)

Chrysoberyl is found in various colours, mostly yellow to green and brown due to iron, light green to bluish green (vanadium) or as the colour-changing variety alexandrite (due to chromium).

In rare cases, chrysoberyl is also found in colourless and chemically rather pure crystals. Recently, the authors had the opportunity to study a tabular colourless chrysoberyl reportedly originating from an alluvial placer in the Mogok Gemstone Tract in Myanmar.
The most interesting feature in this sample were small nail-head spicules oriented along the c-axis of the chrysoberyl, similar to the ones observed in synthetic emeralds and occasionally also in natural gems such as sapphire, spinel, tourmaline and quartz (Schmetzer et al., 1999, 2011; Choudhary and Golecha, 2007).
An examination of these nail-head inclusions with Raman micro-spectroscopy revealed the presence of CO2 in a completely enclosed hollow spicule and secondary iron (hydro)oxide in a hollow tube reaching the surface of the specimen. The colourless inclusions at the ends of the enclosed growth tubes were identified as entrapped chrysoberyl inclusions.