TL, EPR and optical absorption studies on natural alexandrite compared to natural chrysoberyl

The TL, optical absorption (OA) and EPR properties of natural Brazilian alexandrite and chrysoberyl have been investigated. The TL measurements for natural alexandrite show five peaks between 100 and 450 degrees C, with their emission spectrum having 370 and/or 570 nm components. The intensity of the 320 degrees C TL peak was found to be enhanced with pre-annealing treatment, more prominently above 600 degrees C. The OA and EPR measurements showed that this kind of heat treatment induces the Fe2+ --> Fe3+ conversion in the natural sample. Chrysoberyl samples exhibited the TL peaks at the same temperatures as alexandrite samples, but the glow curves were more than 200 times less intense than alexandrite ones.