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 Post subject: facetting question
PostPosted: Tue Apr 26, 2022 10:57 am 
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Can anyone explain the windowing that occurs on the longer sides of oval and cushion cut gems. It seems to occur regardless of pavilion angles and never in rounds. On dichroic material, where the ends are cut steeper to reduce bad c-axis color there is no windowing , but on the long sides there always eems to be this little scalloped area of ,I dont want to say extinction because it isn't black, it seems more like windowing but I've not heard of windowing at edge of stones , only in the center where angles are too shallow. ironically, one oval stone I cut with a centerpoint culet ( requiring some very steep transition angles at the long side girdle) does NOT appear to exhibit this trait. some optical phenomenon beyond my ken , I've reduced the size of the windowed section by squeezing the pavilion angles closer together ( ie: last tier at girdle closer to critical angle- ) but still cant eliminate it. and why doesnt it occur at the short end of oval- even though pav and crown angles are identicl?
Tried to upload a picture file showing this but app wont allow it? Would,ve been very helpful oh well


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 Post subject: facetting question
PostPosted: Tue Apr 26, 2022 11:12 am 
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Trying to upload pic related to most recent post


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 Post subject: Re: facetting question
PostPosted: Sat Apr 30, 2022 1:12 pm 
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Location: San Marcos, CA
Tilt angle windowing, refraction and reflection. It's all about angles and light.
If you were to cut round girdle facets with the long edge angles of the Oval and tilt the round to the same viewing angle you will see the effect to a degree. Probably can be explained more scientifically.
Angles get steeper on ovals near the shorter edge so commonly give a darker appearance, depending on the gemstone, color saturation, closed axis, etc. Oval brilliant and barion designs can alleviate these issues.

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 Post subject: Re: facetting question
PostPosted: Sun May 01, 2022 9:47 pm 
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This is definitely the result of the crown and pavillion angles in the regions being too steep, it can happen in rounds, and is particularly common in quartz portuguese cuts. Of course, on a round it's even around the entire stone. Careful design can significantly minimise it, and many faceters have done so successfully in sapphire.
Image

BobKeller has a good article for a detailed look into this, with images showing the effects, and even how it relates to the concept of the barion cut.
An archived version can be viewed here

Even without all this... the shorter sides of stones provide more chance for a light ray to take a path that isn't a simple "U" shape for total internal reflection, almost all emerald cuts have good tilt performance on the shorter sides, and these more complex paths make marquises look they way they do.


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