The International School of Gemology has an excellent discussion/explanation of the increasing prevalence of fake tanzanite.
Our previous research on this topic turned up prima facie evidence that someone out there (China or Thailand) is color infusing zoisite using the same method that the ISG uncovered being done to tourmaline...Apparently under magnification, the infused color can be seen concentrating in the natural fissures of the stone, as shown here:
But a new treatment method avoids this pitfall:
Rather than using a simple color infusion process, it appears that this new process is using a color infusion material that itself has optical properties that emulate a tanzanite. By infusing a previously colorless or perhaps light yellow zoisite with this optical material the cookers have been able to achieve a level of treatment that surpasses anything we have yet seen on the market...Via the newsletter of the Madison Gem and Mineral Club, which is holding its Gem and Mineral Show at the Alliant Energy Center this weekend. There's always lots of cool stuff on display and for sale at this annual event.
The final test that exposed the condition of this tanzanite crystal was quite unexpected and profound. This crystal, viewed through a London Dichroscope [top embedded image], clearly demonstrated that something very strange has taken place with this crystal. This image clearly exhibits four separate colors coming from this tanzanite crystal, with the blue and purple coming from the fractures and fissures, and the crystal itself offering a colorless and strong yellow reaction. One direction of viewing through a London Dichroscope, but producing four distinct colors. We believe that 2 of the colors are optical reactions from the gemstone, and 2 are due to optical reactions of the treatment material.
...we have to conclude that something artificial is being done to zoisite to create a tanzanite appearing result. Precisely what is being done, we do not know. That it is being done, we believe there is little question left on that issue. The images speak for themselves. What is most remarkable about this find is that the cookers have stepped up their game. They are not simply infusing colors, they have elevated their ability to actually infuse material with specific optical properties.
To the cookers responsible: You folks are brilliant! We need more tanzanite on the market, both natural and treated. But we need your treated material to be properly disclosed because failure to do so is destroying the entire tanzanite market for everyone. Disclose your treatments! Otherwise you will eventually cause great damage to the market for tanzanite just as you did for Ruby, Paraiba Tourmaline and Oregon Sunstone.