Synthetic alexandrite - Growth methods and their analytical fingerprints

Abstract

Synthetic alexandrite crystals grown by various methods (flux, Czochralski, zone melting) and originating from different manufacturers were investigated by standard gemmological testing and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). All samples investigated have (Cr3+ + V3+), or (Cr3+ + Fe3+), or Cr3+ only as colour dopant. The presence of trace contents of iridium ± platinum confirm Russian Czochralski and flux “Cr+Fe-type” samples to be synthesized from commonly used Ir crucibles. Other samples, i.e . Russian flux “Cr+V-type”, Kyocera Czochralski and Seiko zone-melting synthetics, have no Ir, but stand out by consistently present Mo contents. Thus, it can be assumed that Mo containers instead of Pt or Ir crucibles were used. Traces of molybdenum have also been observed in Russian flux “Cr+Fe-type” samples. However, the strongly variable contents of Mo, Bi, and W imply the use of a Bi2O3-MoO3-WO3 flux. These synthetic crystals further show additives such as B, Ga, Ge, and Sn in considerable concentrations. Other randomly encountered “impurities” are Sc, Mn, Co and Zr. Observed inclusion features are mostly in accordance with the respective synthesis method. Flux-grown crystals show inclusions of flux residues of varying texture whereas Czochralski crystals characteristically display curved growth structures. Crystals produced by zone melting (Seiko) typically have swirled or undulating growth structures. Samples received as Tairus zone-melting alexandrites do strongly differ from the Seiko products by their Mo content and different inclusions features. The manufacturing method of these samples is therefore doubted. A proper distinction of natural alexandrite from melt-grown synthetic alexandrite can be done on the basis of infrared features in the range from 2500 to 4000 cm−1.