[Jewelry making - Article 00594] - Re: [Orchid] Fire Agate, Cause of Color?

[The Doctor]

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    I'm not an expert, but a peek into Liddicoat's Handbook of Gem ID
    says that fire agate is a chalcedony with a brown bodycolor and
    strong iridescence from hematite or goethite coatings on mammillary
    (or, if you prefer, botryoidal), chalcedony formations, which have
    been covered by still more chalcedony. That falls in line with the
    "iron theory." 

    Honestly, though, I doubt that it's a theory at all. It would be
    difficult to find a published study, but the information, at least,
    has been published by GIA, and others - Webster, for one. These
    noted gemological luminaries studied all gem materials with far more
    than just X-ray crystallography and scanning electron microscope
    studies, including Secondary Mass Ion Spectroscopy and other
    spectroscopic equipment. I can't come up with a published paper
    about it....yet....I'm having trouble with GIA's EBSCO library. But
    thorough analyses have been done by more than one prominent
    gemological laboratory, and they aren't assumptions. 

    However, even the most advanced equipment can lead scientists
    astray. For example, tigereye was long considered a pseudomorph, or
    quartz replacement. Quite recently, it has been discovered that the
    quartz hasn't actually replaced the crocidolite. Newer, more
    technological equipment has allowed this new discovery. Perhaps your
    friend is on to something. 

    It would be very interesting to know why your friend has reason to
    believe that fire agate's iridescence isn't caused by the materials
    that are currently believed responsible for the phenomenon. If he's
    right, he will go down in history. Will you let us in, or do we wait
    for the press release? 

I'm dying to 
James in SoFl

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