Recently this contributor received a light aqua blue stone that unexpectedly turned out to be spodumene. The following gemological properties were recorded for this 29.59 ct oval modified brilliant (figure 1): refractive indices of 1.661 and 1.676, birefringence of 0.015, a biaxial optic figure, and hydrostatic specific gravity of approximately 3.20. Pleochroism was observed using a linear polarizing filter, which revealed blue, yellowish green, and colorless trichroism. The RI and SG values were consistent with those reported for spodumene in the gemological literature. Magnification of this lithium aluminum silicate (LiAlSi2O6) showed parallel growth features, ripple-like internal graining, and most importantly two cleavage planes along the girdle edge. The identity of the stone was confirmed by further advanced testing. Its Raman spectrum (figure 2), with reference to the RRUFF database (R040050), proved that this stone was in fact spodumene.
Pure spodumene is colorless. Various colors may result from the presence of trace elements such as manganese, chromium, and iron, or sometimes from unstable color centers in its crystal structure. When mined, spodumene sometimes emerges from the ground with a blue-violet or green color that can be unstable in light and fade to pink in a relatively short time, sometimes in a matter of hours. This suggests that the spodumene crystals have been exposed to some natural source of radiation (K. Nassau, Gemstone Enhancement, Butterworth-Heinemann, London, 1984, pp. 162–163). Blue spodumene has also been reported to result from artificial irradiation (G. Bosshart et al., “Colorimetric investigation of unstable and stable spodumene colours,” International Gemmological Conference 2011 Proceedings, Interlaken, Switzerland, pp. 26–30). The Vis-NIR spectrum (figure 3) shows the bands at 543 and 637 nm, which are responsible for the blue color of the stone.
The main source of gem-quality spodumene is Afghanistan; other sources are Pakistan, Brazil, Madagascar, Mozambique, and the United States. While it is unknown whether the color of this blue spodumene was of natural or artificial origin, such stones are rarely seen as faceted gems, and this unusual large example would be of interest to any gem collector.
Source: gia.edu Forozan Zandi
