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Re: [Orchid] Tanzanite fake or real?
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The Doctor Sunday, October 23, 2005
   
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>         would a simulated tanzanite... lets say made of glass, have the
>     same red flashs, color change etc. as the real stone would? Anyone
>     with some insite on this subject?

Hi Sheila,

    It's true that tanzanite has not been synthesized in a laboratory
    and, as you've certainly read on this forum, there are a few
    imitations. I wish there were a simple layman's answer for you but
    the truth is that it is difficult to separate some of these
    materials without at least some basic gemological training and
    equipment. 

    For instance, zoisite (tanzanite is the blue variety of zoisite) has
    a refractive index of 1.69 to 1.70, a biaxial positive optic
    character, specific gravity of 3.35, birefringence of.008 -.013 and
    is generally inert under longwave ultraviolet light. This is the
    basic info you need to separate it from its'imitations. 

    Coranite is a trade name for one tanzanite imitation. It is a
    purplish blue synthetic corundum with a typical RI of 1.762 - 1.770
    and a birefringence of 0.008. Its optic character is uniaxial
    negative, which is quite different from tanzanite and it is inert to
    UV light. 

    Some manufacturers have created a bluish-purple yttrium aluminum
    garnet (YAG) that looks a lot like tanzanite called Tanavyte. It is
    singly refractive with an RI of 1.80, which is over the limits of
    most gemological refractometers. Even so, it may show anomalous
    double refraction in the polariscope due to strain in the crystal,
    but that also means there is no birefringence or pleochroism. 

    U.M. Tanzanic is leaded glass with an RI of around 1.60, and also
    singly refractive. Tanzation is a synthetic spinel triplet with a
    fused cobalt layer. RI is 1.72 and it's also singly refractive, so
    no birefringence or pleochroism. 

    You've doubtless read about the synthetic forsterite here, and it is
    definitely the most difficult tanzanite imitation to separate, even
    though it is most closely related to peridot than to zoisite. It's
    optic character is biaxial positive like tanzanite, but it's RI is
    significantly lower at 1.64 - 1.67. That also means it's
    birefringence is much higher at.035. It is strongly pleochroic like
    tanzanite, but only shows two colors through the dichroscope,
    whereas tanzanite displays three colors. Earlier, I mentioned that
    tanzanite is inert to ultraviolet light, but synthetic forsterite
    often fluoresces a weak, chalky orangy yellow when exposed to
    long-wave ultraviolet radiation and a weak greenish yellow to
    short-wave UV. 

    Sheila, as a person who has recently started collecting gems, I don't
    expect you to be able to take the kinds of readings or even own the
    equipment necessary to make the separations required of a gemologist.
    What I am attempting to do is get people to understand how easy it
    can be to get ripped off by unscrupulous dealers, or even ignorant
    ones. Internet sites and legitimate gem dealers often have no idea
    that they themselves have been hoodwinked into buying imitations or
    synthetics before they resell them to the public. I guess what I'm
    saying is that you should find yourself some reputable dealers,
    wholesalers, retailers, etc., who you may buy from with confidence. 

    All of the above is a long, unnecessary narrative to answer the
    question of whether there is a way to tell with the naked eye if a
    gemstone is tanzanite or one of its'imitations. Apart from some
    batches that "just don't look quite right" the answer is: 

    Nope, sorry :-( 

James S. Duncan, G.G.
James in SoFL


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