[Jewelry making - Article 00016] - Re: [Orchid] Gemstone "windows"

[Dave Arens]

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Hi Dan,

>          This gem has good depth & no window..." and "This gem was
>     faceted a little shallow, giving it a slight window and making it
>     look like a much larger one carat gem. 

    What's being said here is that when looking at the stone through the
    table, the stone with the 'window' acts just like a window, you can
    look right through the stone & out the pavilion ( the bottom of the
    stone). The other stone has it's pavilion cut at the proper angle(s)
    so the light striking the the pavilion is reflected back to the
    viewer. It may not come directly back, but may be bounced from two or
    more of the other facets of the stone. 

    The action of light rays within a transparent stone is governed by
    the refractive index of the material the stone is made of. There's a
    'critical angle' that depends on the refractive index. The lower the
    refractive index, the larger (toward 90 deg) the critcal angle is. If
    a stones pavilion is cut below this critical angle, no light will be
    reflected, you'll be able to look right through the stone, hence the
    term 'window'. 

    The formula for determining the critical angle is: 

    sin c = 1/ri 

    Where c = critcal angle ri = refractive index 

    Dave 
 

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