[Jewelry making - Article 00045] - Re: [Orchid] Light entering faceted stones

[Downey]

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    Howdy Orchidians, 		Here's a little test some of may be able to
    perform. We must begin by trying to define a 'well cut stone'. If
    the 'main pavilion facets' (on some cuts this is hard to define but
    is almost always the collection of facets that form the culet point
    or edge or line) are cut at angles that fall roughly between the CA
    (critical angle) and  .333333 times (180-CA) for the gem material in
    question, very little light falling on the 'backside' of the stone
    can be refracted out the front. 

    		In practice, most careful faceters will select/create/modify cut
    designs to yield main angles which are 1-3 degrees above CA. When a
    stone's main pav. facets are cut 'too shallow'(at or below the CA)
    you get 'windowing' or 'fish-eyed' stones. Most folks consider that
    a poorly cut stone (except perhaps for very dark garnets or some
    other criteria REQUIRING an effort to bring light in from the rear).
    The other extreme, too 'high' (steep,deep) angled main pav. facets
    are often found on 'belly cut' stones. Remember that many (often
    foreign) cutting houses will place a somewhat greater emphasis on
    recovery (carat weight) and 'calibrated size' than brilliance and
    ,while sitting under bright lights in a case almost all stones look
    dazzling, the human eye is 'saturated' with 'dazzle' and the true
    performance of a well cut stone may only be noticed in
    'office'/'grocery store'/'intimate' lighting conditions. Nowadays
    few folks are wearing their nice stones in full daylight. 

    		Back to the test. If you can get examples of the 3 categories just
    mentioned, a belly cut stone, a fish-eyed stone and a well cut stone
    proceed as follows. 

    		The fisheyed stone is easy. take a scribe or even your finger and
    pass it back and forth behind the stone. You clearly notice large
    images/shadows moving around directly under the table. There will
    probably be smaller reflections distributed around the crown as well
    but the main 'problem' is the too shallow pavilion angles. This is
    often what happens to beryl/quartz and other low RI materials when
    they get some skin oils on them. Look at a well cut quartz that is
    very clean. Now rub a little 'nose grease' on the pavilion and note
    the difference. Due to a narrow difference between the CA and
    1/3(180-CA) low RI stones are most severely affected by oils or
    other contaminants this way. CZ ,diamonds etc. can still look pretty
    good when dirty. 

    		Now let's take a 'well cut' gem. Use a scribe or narrow probe and,
    while looking at the right side of the crown, move the scribe up and
    down while holding it just over the left side of the crown. You will
    see the refracted image of the scribe's shadow move around (mirror
    image mostly) on the opposite side of the stone. This proves that
    most (not all - more later) of the light entering the stone is
    falling onto it's crown. Now, if you get too close, there can be a
    problem we call 'head shadowing' and some otherwise nice looking
    stones do not 'accept' light from a very broad 'cone' around the
    crown, but in general they still are better than the other 2
    'categories'. If you extend the scribe all the way to the center of
    the table, the refracted images should still be seen. If you perfrom
    this test with the fisheyed stone you probably WILL see the scribe's
    image in the break/main/star facets (because they plus the
    pavilion's BREAK facets may be at 'working' angles) but you will not
    see the image refracted by the facets under the table. The light is
    'falling through' or we could say only light entering from behind
    the stone will enter the eye from the fisheyed table. 

    		Now, something just a little different is happening with the
    'belly cut' stone. Light rays entering the pavilion facets from the
    side but down at an angle (let's call it 45 degrees, but it's a wide
    range in reality) are making one 'bounce' off the opposite pav.
    facets and exiting the table to land in your eye. In a well cut
    stone most rays enter the crown (top) are bent slightly, bounce off
    2 facets and exit the crown (bent slightly again) hopefully headed
    into the eye. So, move your scribe around the side (and perhaps down
    a little) of the pav. of your 'bellycut' stone and notice
    reflections of the scribe through the table. They will most like ly
    be on the pav. facets OPPOSITE the side on which you are moving the
    scribe. 

    		Here are some caveats (actually cover my a**<g>). There are 
many
    types of cuts cut in many ways in many materials. Some cuts are
    notoriously poor performers (optically) though they may sometimes be
    chosen for reasons of tradition ('emerald' cuts) or style
    (navette/marquis). Even many ovals perform poorly in low RI
    materials. The best way to do the above tests would be with SRBs in
    a medium RI low dispersion material - say - white sapphire. But,
    some ovals may be OK to use. though, when we call a cut 'poor'
    performing we mean it has low brilliance DUE TO EXACTLY THE REASONS
    STATED ABOVE!. Too many 'areas of extinction' where a light ray
    would HAVE TO enter the stone from the girdle or the back to arrive
    at your eye! Most jewelry either has limited openeings in the back
    or is worn such that the opportunity for significant amounts of
    light to enter the back is nil. 

    		I alos apologize if using terms like up and dowm ,top, back etc.
    are confusing. I really am using my hands to show you!<g> I hope
    someone will try some of the tests and see if they are helpful. Do
    keep in mind that, considering their pay scale and equipment,
    foreign cutters do a good job. And if the incentive were shifted a
    little more to brilliance (which I believe it has in the last
    decade) than recovery,  it would be a noticeable improvement. The
    more you educate your selves, the more they will respond to demnads
    for better cutting. Carl 1 Lucky Texan

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