[Jewelry making - Article 01263] - [Orchid] Re: Facetted stones and reflective backings

[jake]

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>     I think you mean 'refractive' index, not 'reflective'. The RI is
>     the amount the light will bend when entering a material. The RI is
>     what determines the Critical Angle of the material. It is the
>     critical angle that determines the light return. The lower the CA
>     the more light enters the stone as opposed to be reflected off of
>     it. These factors also determine the best angles for pavilion and
>     crown facets to maximize the material. Another factor that will
>     affect light return is the type of cut. Barions normally offer a
>     better light return than standard cuts.

Craig

    I see you caught a spelling mistake. (Had my mind on one tract.) How
    light is captured and reflected is what gives a gem its sparkle. 

    The refractive index (RI) is the measure of the amount light is
    bent. Determining the critical angle. Above the angle light is
    reflected back, below light passes through, resulting in windowing.
    This determines how facets should be placed in order to control the
    path of light so it will be internally reflected to best effect. The
    RI, quote, "also tells how much light will reflect off a boundary"..
    (Physic formulas do not help me at all. Not sure they really even
    impart understanding in those who can read it.) 

    On optical performance the RI also determines (all things being
    equal) the intensity of light returned to the viewer after entering
    the stone and reflecting off the facets (cut planes) and returning
    out. Higher RI stones are better at that than lower RI stone. Why a
    high RI stone that bends light more also reflects the light back
    better (as noted in many places) I have no idea. Only all can see the
    result. Which is why white sapphire will be brighter than clear
    quartz (rock crystal). Even if both are cut for optimal effect with
    best angles, if both were barions of equal clarity and polish etc.
    the sapphire will be brighter. That was what I meant to convey. 

    Of course, usually it is the cut, or lack of that has far more
    effect on appearance than other factors. This is the single most
    important factor. (Providing it is worth cutting or as said somewhere
    about some Ebay material, if light can't pass through it, it is not
    faceting grade, and other remarks.) Even the best material if cut
    wrong will have disappointing results.  Thai cut gems are notorious
    for that, quick and cheap, lack of good polish (usually seen) adds to
    that result. The optical performance is often not there and sometimes
    more or less completely lacking.  Mention has been made to Swarovksi
    leaded glass rhinestone. That 30 % lead glass has a higher RI than
    quartz (RI, 1.544 -1.553). They are also machine cut to best angles.
    I could not tract down the exact RI for Swarovksi cut glass, I
    believe it is 1.62. White sapphire was said to be 1.80. When trying
    to find the exact Swarovksi glass RI out of curiosity I came across
    this. "The CUT of the rhinestone greatly influences its brilliance!
    Purchase the rhinestone with cut in mind." "Do not be deceived by
    pricing and think that 3 or 4 times the number of rhinestones applied
    to a garment will make the garment outstanding IF you use an
    inexpensive rhinestone."

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