Glass has long been used to imitate a wide variety of gem materials. In recent years, we have seen an increased amount of blue to violet glass being used to simulate the very popular gem tanzanite. The West Coast GIA Gem Laboratory recently received a 5.42-ct. transparent violetish blue oval modified brilliant for identification (see figure). The specimen, submitted to the laboratory by Martinek’s Jewelers of Traverse City, Mich., was brought to them by one of their customers, who had purchased it in Mexico as tanzanite. Upon recording the gemological properties, we saw how a quick examination could lead to a misidentification.

The most interesting characteristic of this material was its high refractive index (R.I.), which was very close to that of tanzanite. We recorded a single R.I. of 1.700, which is the high value for tanzanite (1.690–1.700). The glass imitations of tanzanite we have examined in the past have seldom resulted in R.I. readings that exceeded 1.66. The fact that the specimen was singly refractive (with weak anomalous double refraction) was the main distinguishing property separating it from tanzanite, and this also meant that it lacked tanzanite’s characteristic pleochroism. Additional features of the stone included a calculated S.G. of 4.11; weak blue fluorescence to long-wave UV radiation, and weak-to-moderate chalky yellow and blue fluorescence to short-wave UV; as well as weak bands seen at approximately 500 and 600 nm with a desk-model spectroscope. The stone was fairly free from inclusions, and magnification revealed only a few pinpoints.

Both Fourier-transform infrared spectroscopy and Raman spectroscopy provided spectra similar to those previously recorded for manufactured glass, which confirmed the identification.

The important point with an imitation such as this is one that we continue to mention on a regular basis: Always use all the available information in making an identification. A quick R.I. reading coupled with the violetish blue color could easily lead to the wrong conclusion. However, if tanzanite is suspected, the characteristically strong trichroism should be readily detectable if the material is genuine.

Elizabeth Quinn
GIA Gem Laboratory, Carlsbad