[Jewelry making - Article 01308] - Re: [Orchid] Distinguishing synthetic from natural opal?

[The Doctor]

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    Hi Jeanne. If the seller's parents actually bought the parcel in
    question in the 70's it is certainly possible that your opal is a
    synthetic, and possibly Gilson, since Pierre Gilson invented it in
    1974. Still, it wasn't very widespread at that time, so it is also
    possible that it is natural. 

    Gilson has done such a good job in creating their opal that it is
    difficult for the untrained eye to separate it from natural opal.
    There are, however, some differences that a trained eye can spot.
    The most prominent giveaway is something known as the "snakeskin
    effect" that is best seen with a microscope, but may also be
    detected with a loupe. Look closely at a patch of color and try to
    determine if it has a scale-like appearance. It's easy to miss if
    you don't know what to look for. Also, the play-of-color of most
    synthetic opal has a columnar structure when viewed from the side,
    depending on how it was oriented when cut. As a self-avowed
    opalholic, I've looked at a lot of them under magnification, and I
    have seen both "snakeskin effect" and columnar structure in natural
    opal, as well. It can be difficult to tell the difference sometimes
    between natural and synthetic. 

    Another thing is fluorescence and phosphorescence. If you have
    access to longwave and shortwave UV light, darken the room and check
    the stone with them. If it (a synthetic) has a white base color, it
    may show moderate blue to yellow fluorescence and no phosphorescence
    (glows a bit after you turn off the light) under longwave. Under
    shortwave, it may show moderate to strong blue to yellow
    fluorescence and weak phosphorescence. 

    If a black base color, it could show anywhere from no reaction to
    moderate yellow with no phosphorescence under LW, and none to weak
    yellow under SW. If it is a true crystal base as you say, it
    probably isn't synthetic, but many Gilsons appear to be crystal, but
    aren't. Also keep in mind that batches of synthetic opals vary in
    character quite a lot, and all of the above may or may not apply to
    yours. It takes some experience to separate certain synthetics from
    natural. 

    There are also minor differences in refractive index and specific
    gravity, but they overlap and can be confusing and I'm guessing that
    if you had a refractometer, you wouldn't be asking the group. 

    I am always loath to ID a gemstone from a photo, but some Gilson
    does have a telltale structure. If you have a pic, let me know
    off-list. But don't count on it. 

James in SoFl

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