[Jewelry making - Article 01141] - Re: [Orchid] On pavilion coatings...

[The Doctor]

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>          What I can't figure out is....since the coating is on the
>     *outside* surface of the facets in the case of both stones, how
>     come the light coming into the stone through the table and upper
>     facets and then back through the stone changes color....? 

    Gary, once you change even the surface color characteristics of a
    gem material, you also change its absorption pattern, or the way it
    absorbs (and reflects) light, even if in a small way. Take natural
    green diamonds, for example. Nearly all natural green diamonds
    experienced some radiation after formation, but before being brought
    close to the earth's surface via a kimberlite or lamproite eruption.
    As a result, the radiation causes a modification to the diamond's
    crystal structure that is virtually always confined to the surface
    "skin" of the rough. These diamonds are usually entrusted to the
    finest cutters, who leave as much of that green colored skin intact
    at the girdle so that the naturals left behind will contain the
    green color that is reflected (hopefully) throughout the diamond. 

    What happens after irradiation is that the electrons in the carbon
    atoms near the surface skin of the diamond behave differently, so
    they absorb light differently. A good parallel example is how
    amethyst becomes citrine when heated. Most people in the trade
    understand that quartz is Silicon Dioxide, or SiO2, and that
    amethyst contains trace amounts of iron (Fe) that replaces some of
    the Oxygen. This is called a "color center" and it changes the way
    the quartz absorbs light. What happens is that neighboring electrons
    (called ions) transfer between the Iron and Oxygen atoms, creating
    an intervalence charge transfer that causes the quartz to absorb
    most colors except the red and blue (mostly yellow, green and
    orange) that our eyes view together as purple. Radiation will remove
    electrons from the iron impurities, which causes more Oxygen - Iron
    intervalence charge transfer, which in turn intensifies the purple
    color. Heating the amethyst destroys the color center, which again
    changes the absorption pattern to yellow, or citrine. As long as
    there are still traces of Iron, irradiation will remove Oxygen from
    the Iron atoms and return the purple hue. 

    So, even if you create a color center only on the surface of a
    faceted stone, it's still a color center and can reflect throughout
    a well cut stone but will usually be pink"ish" or green"ish" as
    opposed to a nicely saturated gem. This is why people should beware
    of intensely vibrant green diamonds...few natural greens are vivid. 

    In rare cases, such as the Dresden Green diamond, the rough was
    exposed to the radiation long enough so that the color is saturated
    throughout the stone, but that amount of time is only speculated
    upon...nobody really knows...possibly millions of years. This is
    often a clue for gemologists to separate natural greens from the
    human-irradiated green diamonds I see at gem shows these days. That,
    and the extremely low price of irradiated stones. True natural
    greens are very rare and very expensive. 

>         Does the coating act as the new surface boundary re:
>     reflection on the pavilion...? 

    Not so much a new surface boundary as a new "trace element" or
    "color center." Obviously, it isn't absorbed into the stone as a new
    element, but it *acts* like one in that the colored ink from the
    sharpie marker is in contact with the material, and so changes the
    way it absorbs light. This is the reason gemologists are cautioned
    during training to make sure that a stone being color graded doesn't
    touch the masterstone or color sample by which it is being graded.
    It's also why masterstones are usually accompanied by one white- and
    one yellow-colored temporary mount for their masterstones. It is to
    mount the masterstone in the same colored metal in which a mounted
    stone is to be graded. Basically, a stone mounted in yellow gold is
    compared to a masterstone that is temporarily mounted in a yellow
    mounting to help offset the difference in color the mounted stone
    may pick up. 

    Sorry for being so lengthy but I still left a lot out of the
    equation, even so. I'd be happy to muddy the waters further on- or
    off-list to anyone. 

James S. Duncan, G.G.
James in SoFl

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