June 8, 2017 in articles

GLASS IMITATION OF TANZANITE

By Tay Thye Sun & Loke Hui Ying

(Published in the Journal of Gemmology, 32(2) 2014, pg 109-110)

Figure. 31. These attractive purplish blue gems proved to be glass, rather than tanzanite. The cushion cut on the far right weighs 8.83 carats. Photo by Tay Thye Sun.

 

In June 2014, a jeweler submitted a group of three loose gems and a man’s ring containing what looked like tanzanite (Figure. 31). The ring consists of 18k white gold, and the centre stone was surrounded by one-point diamonds.

Upon initial inspection, the purplish blue colour of the material showed a strong resemblance to tanzanite. However, testing of the 8.83 ct cushion cut showed it to be singly refractive, with a RI of 1.705 and a SG of 4.13. With magnification and transmitted light, the gem was clean except for swirl-like structures (Figure. 32). The presence of abundance polishing marks (again, see Figure. 32) and some scratches suggested it was a relatively soft material. It was inert to long-wave UV radiation, and fluoresced a weak chalky white to short-wave UV. No absorption features were evident with a desk-model spectroscope. These feature are similar – but not identical – to those documented for a high-RI glass imitation of tanzanite by Quinn (2003). Fourier-transform infrared spectroscopy confirmed the identification of this material as artificial glass.

Although this type of tanzanite imitation has been known for more than a decade, it continues to circulate on the market. It is also interesting to note that even fine jewellery may contain such glass imitations. Therefore, it is important to exercise vigilance and always be sure that a ‘tanzanite’ gemstone has a strong pleochroism (and other expected properties) characteristic of this gem material.

Figure. 32. Swirl marks were evident in the glass (particularly on the left) when viewed with magnification and transmitted light. Polishing marks are also seen on some pavilion facets. Photomicrography by Tay Thye Sun; magnified 15x.

Reference: Quinn E., 2003. Lab Notes: High-RI, glass imitation of tanzanite. Gems & Gemology, 39(4), 317318.

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