Gemmological testing of tanzanite
Tanzanite is a soft stone, measuring 6.5 on the Mohs hardness scale. The dichroscope is a great help in identifying loose and mounted tanzanites, as the gem has characteristically strong pleochroism (i.e. a crystal exhibiting different colours when viewed from various angles). Only unheated tanzanite exhibits trichroism (namely blue, purple, and yellow), whereas all heated stones are dichroic, displaying blue and purple. Purple is a modified spectral hue that lies halfway between red and blue.
Figure 1 shows the difference between heated and non-heated tanzanite using UV-VIS-NIR spectra. The main criterion is a transmission ‘window’ in the shortwave UV range. It is generally beneficial to analyze along the blue pleochroic direction to see the transmission window the best. After Notari et al. (2001), it also became possible to measure the spectrum unpolarized; if the stone is heated, it transmits light from 300 to 400 nm in the UV part of spectra.
At this time, we can suggest the identification of natural-coloured tanzanite is far more difficult than what some gemmological publications would have us believe based on UV-VIS-NIR spectroscopy criterion proposed many years ago by European researchers.
The question for traders and jewellers remains whether they should disclose tanzanite as heated or just state that it is ‘natural tanzanite’ at the point of sale and say it is not possible to prove or disprove heat treatment. Our policy is to indicate ‘Not detectable’ under ‘Enhancement’ on the report, with the additional qualifying statement that ‘Most tanzanites are heated to improve colour.’ In the rare case where we can prove natural colour, we state that on the report.
Gemmological testing at most labs may not be fully conclusive with current technology, so further research into the testing of non-heated material should be a challenge worthy of consideration for top labs. Perhaps a spectra database would be a good starting point.
“The laboratory grading of tanzanite is still in its early stages, and the tanzanite industry as a whole is not sophisticated enough to absorb a market that requests natural-coloured tanzanite as a preference,” says Hayley Henning of TanzaniteOne Foundation. “We are starting to see this, however, but in general, consumers are still obsessed with violet-blue tanzanite, which is generally known to be heated as a part of bringing out the natural beauty of the stone.”