[Jewelry making - Article 00214] - Re: [Orchid] Frustrated with final designs

[Peter W . Rowe]

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>       Peter, I think you would be hard pressed to demonstrate light
>     entering the pavilion of any well cut stone. 

    I belive that's exactly what I said.  In a well cut stone, the
    viewer does not see light that has entered the pavilion of the stone.
     however, if the stone is trasparent, light does indeed enter the
    pavilion, just as, if it hits any other facet, like the table,it also
    enters.  It's just that the light that enters the pavilion generally
    does not exit through the table or crown in a direction which is
    visible to the viewer from a table up position.  But, for example,
    take any well cut diamond, and view it from the side.  You can see
    right through the pavilion to the other side of the pavilion.  Not
    clearly, of course.  But move a needle behind the stone and you'll see
    partial broken images of the needle through the stone.  Thus light is
    entering the pavilion, in this case, viewable also through the
    pavilion.  You'll see more of this if you look, from the pavilion
    side, into the stone towards the inner surface of the table.  In that
    surface, you also will see reflected light that entered the
    pavilion.  Again, it doesn't affect the table up appearance of the
    stone very much, but if you intended to say that no light enters the
    pavilion, that statement is a bit too broad, unless you're talking
    about a well cut foilback (grin). 

>       ... In the last paragraph you mention colored stones with lower
>     refractive indices.  Just for those who may not know:  if you stick
>     a stick in water it looks like it's broken where it enters the
>     water.  That's because light travels at different speeds depending
>     on the medium it is passing through. Many of the stones ( crystals)
>     we use are cut with this in mind.  A stone properly cut will not
>     allow light to leak out.  On the other hand, a windowed stone is
>     improperly cut and will allow light to leak out of the pavilion or
>     other places diminishing the brillance of the material.  The "lower
>     refractive index" is not better than a high r.i.; it's just
>     different. 

    Each stone can be appreciated for it's own merits.  But those with a
    higher refractive index allow a wider range of pavilion angles to be
    used without getting a window or an area of extinction (another type
    of light loss, where instead of seeing through the stone, you just
    see a dark shadow.  It's due to too steep a pavilion angle instead of
    the too shallow angle that gives a window) Because of this, as a
    general rule, stones with higher refractive indices are usually
    capable of being cut with greater brilliance (light return).  As you
    say, this is not necessarily better, only one of the many factors
    that makes each stone a unique type of material. 

>         Each stone, that has a crystalline structure, has an
>     appropriate refractive index because of the way light behaves in
>     that particular material. These comments apply only to stones that
>     have a crystalline structure and to the oddball opal that can
>     possibily be faceted. 

    Here I must correct you.  Materials don't need a crystal structure
    to have a refractive index.  Clear glass, with no crystal structure,
    has a refractive index.  so do transparent liquids.  What you're
    referring to is the fact that those crystaline materials which have
    crystal systems OTHER than cubic, because of the fact that their
    lesser symmetry means the optical environment can be different
    depending on the direction through the crystal that light travels,
    or depending upon the vibration direction (polarization direction),
    light may travel at different speeds within the same material.  That
    means that the refractive index can vary with direction, or
    polarization direction of the light.  Such materials are known as
    double refractive, and they do something that non-crystaline
    materials, or cubic materials (except when strained) don't do:  They
    split light into two polarization directions as it enters ion any
    direction other than directly along an optic axis, and each
    polarization direction has it's own refractive index, as well as,
    sometimes, a different color.  The result is that gemologists have a
    whole bunch more information that can be obtained by measuring the
    refractive index of a material, or more importantly, the appearance
    of a stone can do wonderous things, like showing double images though
    the stone, or two or three colors to the same stone, at once, through
    different directions.  It's why stones like ruby can be puplish red
    in one direction and orangy red in another, or a long thin topaz can
    be pinkish peach at the ends and yellower color in the middle, as
    light travels across the stone for the latter color, and end to end
    for the former.  Or try Tanzanite, which before the heat treatment
    that leaves it nice and purple, is reddish in one directly, blueish
    in another, and yellow in a third (heat treatment removes the yellow,
    so the overall muddy brown color of the untreated stone becomes a mix
    of the blue and red directions.) 

    cheers 

Peter Rowe 

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