[Jewelry making - Article 00104] - Re: [Orchid] Gemstone "windows"

[Downey]

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    Howdy Dan, 	

    I'm sure others hear will respond too. 	 

    Almost all cut stones will window if tilted enough. The problem of
    course occurs when, in a mostly face up position, there is no
    refracted light coming from the main facet area under the table. If a
    gem has it's main pavilion facets cut 'below' (numerically less than)
    its critical angle (determined by the refractive index) then light
    can only reach your eye from that area if it enters the back of the
    stone.(ie. "Wow! I can see your finger through your ring's stone!")
    If the pavilion mains are cut above (180-CA)/3  then you may have
    what some folks call a belly cut (there is some disagreement on this
    term?) and light can only exit the table after entering the side of
    the pavilion and reflecting off one main facet to then shoot up and
    out. The goal is to create a stone that has as much of the incident
    light falling on the crown returned back out to the observer's eye.
    There are of course compromises and complications. head shadowing,
    color saturation, pattern/distribution of scintillation/reflections,
    certain odd feature enhancements like ametrine, etc. Also, as RI goes
    up this is less of an issue. I'm not sure you'd ever hear of diamond,
    CZ or maybe zircon exhibiting windowing. But most other colored
    stones could. 	Most hobby faceters strive to minimize windowing
    except possibly if dealing with extremely dark materials (garnets-see
    haiku below<g>) where it's thought to help get apparent brightness up
    by allowing windowing. As to value, well that depends on folks
    expectations and the design goal of the cutter I guess. Personally,
    I'm ashamed of the early stones I cut which window with a very slight
    tilt but there is one my wife loves because of the 'twirling dance'
    the window does around the pavilion. Go figure. 	 

    Choose rough carefully; 		 

    Earth's dark unconquered by man. 		Garnets yield no light. 

Carl
1 Lucky Texan 

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