[Jewelry making - Article 00213] - Re: [Orchid] Bezel setting faceted stones

[The Doctor]

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>>         You can read print through some stones more easily than
>>     others, no matter how well its angles have been cut. 

>         Thank you so much for sharing more info on this. Your sentence
>     above has me a bit confused. I was told the window effect is
>     occuring because the facets were not cut at the correct angle.
>     Sounds like you are saying a stone can window even with the facets
>     cut correctly. Am I misunderstanding you? Is this not something I
>     should check for as a sign of a well cut stone, along with
>     proportion and polish? 

    Hi Carrie. Yes, you definitely misunderstood me. If you'll re-read my
    post, you'll see that I was talking about the "see-through effect,"
    not windowing. "See-through" is observed by placing the stone, table
    down, on print. Windowing and extinction are both seen face-up, and
    reading print through the stone has nothing to do with it. Instead of
    my saying "Look for the chapter on the "see-through" effect," I
    should have just explained it, and I will. But you'll also see later
    in this post that even with correctly angled facets, windowing and
    extinction do still occur to some degree. 

    First: See-through. See-through is a practice used by gemologists to
    lend a clue mostly (but not exclusively) to separate diamond from
    its simulants. If you have a round brilliant cut diamond and similar
    CZ, place them, table down, side by side on some printed material.
    With diamond, you won't be able to read the print. With CZ, you will
    be able to see the print through it slightly. GGG has moderate
    see-through, and YAG has strong see-through - you can read just
    about anything through it. This has to do more with the material's
    refractive index (RI) and the way it bends light than it does with
    how well it is faceted. Of the four gems I mentioned (diamond, CZ,
    YAG and GGG), diamond's RI is highest, GGG's is lowest. Try some
    other gem materials and see how easy or difficult it is to read the
    print through them. The lower the gem's RI, the easier to
    "see-through." 

>         I have come across many stones that have this problem
>     (windowing) in varying degrees, though I did not have the
>     terminology to describe it until now. I just knew that something
>     was not right. 

>         My book also talks about extinction in a stone. I am finding
>     this harder to see as there seem to be dark places in the facets in
>     most stones as you move them around in the light. Maybe I just need
>     to view a stone in person that has this strongly and then I'll get
>     it. 

    Practically all faceted colored gemstones have at least a small
    amount of windowing and extinction. The (very) light and dark places
    you mentioned are just that; windowing and extinction. The balance
    is referred to as light return. If a gem has 25% windowing and 25%
    extinction, it follows that it has 50% light return. The percentage
    of light return is used to judge the stone's brilliance. Generally,
    any gem with over 75% light return is said to have excellent
    brilliance. Texts that address windowing and extinction without the
    rest of the equation can be confusing. Photographic examples that
    are usually given in textbooks for those phenomena are pretty much
    always extreme. Notice I typed (very) light and dark above. Very
    light means areas that are lighter than the bodycolor of the stone.
    Very dark...well, you get the point. Theoretically it's possible to
    cut a gem of any material with 100% light return, and I'm fairly
    certain that if you could, it would be a very uninteresting gem. In
    reality though, it is impossible to do so. And thank goodness,
    because without a certain amount of windowing and extinction, there
    could be no scintillation. 

    When you hear or read that a stone is windowed, it really means that
    there is an unacceptable amount of windowing in it, not that the
    entire stone is a window. Ditto for extinction. That is, unless the
    stone is cut into a cube. I reckon that'd be a fully windowed stone. 

    The fact is, windowing can lighten dark materials such as some
    garnets, chrome diopside, some sapphires, etc. Without a certain
    amount of windowing, these and many other gemstones are simply too
    dark to be attractive as a faceted gem. Also, extinction is
    manipulated by skilled lapidaries for materials with pale,
    unsaturated colors such as some aquamarine, topaz, etc. To find
    examples of windowing for your own curiosity, look for gems with
    very shallow pavilions. For extinction, check for deeper pavilions.
    Other proportions come into play too, as well as the material from
    which the gem is cut. Extinction in gem cuts with an aspect ratio
    (i.e. 10X6, and more so, a wider ratio like 10X3) such as oval,
    marquise, etc., usually show the "bowtie effect." You'll recognize
    it the moment you see it next. I'll bet you've seen it before, but
    just didn't know what it was, or what to call it. Look for a really
    long marquise or oval, you'll likely see it. 

    Hope I didn't confuse you further. If so, I'll be happy to confuse
    you even more :-) 

James in SoFl

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