15 November 2012

Be aware of fake tanzanite

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...

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.
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.


  1. As a goldsmith with years of experience, this doesn't surprise me in the least. Fakery and trickery with gemstones has been around as long as there has been a market for them. Faience and "paste" jewels were invented thousands of years ago, apparently in an attempt to fool the guardians of the afterworld - and to save the expense of burying the real thing with the deceased!
    Pretty well every coloured gemstone you'll buy today has been doctored in some sense - oiled, steamed, dyed, chemically-treated or what have you. Most consumers would never know the difference. Indeed, most gemologists would need to test for a stone's refractive index to verify a genuine or find a fake.
    Probably best to assume you are NOT getting what you're paying for, unless the vendor is willing to provide a certificate or some other form of proof. Which begs an interesting question: If you're paying for a pretty purple rock, and you get a pretty purple rock, then what are you really paying for - the stone or the romance associated with the stone? This is why I often will try to convince clients of mine to go without a stone in their design - or else make it very clear to them that no stone is likely to be what it purports to be. Or if it is, it has probably been doctored in some way.

  2. "or else make it very clear to them that no stone is likely to be what it purports to be. Or if it is, it has probably been doctored in some way."

    There's lots of coloured gems out there that are not treated for example many tourmalines, garnets, spinels, certain colours of zircon to name just a few. Although many sapphires and rubies are treated, some to the extent as being termed "assembled", it is possible (for a premium) to still get material that is untreated. To say that all stones are likely "doctored" is not true.

    1. Perhaps I should have said "no stone is guaranteed to be what it purports to be". But I stand by my assertion that most stones are somehow treated. Even something as "non-invasive" as heat treatment is still treatment. Customers just don't want watery-looking or murky, heavily-included stones - even semi-precious (ie. cheaper) ones. Even marginal garnet rough that, years ago, would have been consigned to making sandpaper is now being pressure/vacuum-dyed to make it passable as faceted material. Of course not ALL stones are treated; but many more than people would ever imagine certainly are.


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