Why do some diamonds look dark inside?
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strmrdr
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thinner arrows, brighter under the table, works better with the crown angles that I like the personality of the best.
In rounds for me the brighter the better while maintaining light show ability that means super ideal diamonds with low 34.x pavilions and high 40.x(41.0 as long as none are a lot over is good too) with longish lgf and long stars look the best to me.
Id rather have an asscher :}
strmrdr
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34.4 and 40.8 is an awesome combo but borderline my favorite.
keep in mind I wouldnt recomend not buying a 34.4 and 40.8 combo if everything else was right its a kicken combo.
As for the lgf it depends on what looks good to your eyes.
I wouldnt call it dark, ACA''s are cut to 77-79 and an occasional 80 and a lot of people love them.
Its not a matter of cut quality when it gets down to this level its about personality and in most cases the differences are small in a lot of light conditions until you move in a large direction one way or the other.
For example pretty much anyone would notice the difference between 75 and 78 and 81 lgf height.
Where pretty much no one will see the difference between a 79 and 80.
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Good Q... I tried to figure out what other parameters need to be correlated with a certain LGF to me it 'work'.
Strm, from your answer to this one it seems that the crown angle is it and not much of anything else. Is it so?
My intention was a bit different - I like OEC's look and that calls for short lgf as a key ingredient. For better or worse, those cease looking 'dark' only with a pretty high crown above and/or not much of a table if there is to be some limit for how steep the crown gets. This suggests some trade-off between thin lgf and crown angles... but I have not went on to check this (& reinvent the wheel).
Anyway, of course I wouldn't like an 'ideal cut' with a sink hole under the table - and thin lgf (as thin as you'd want them) prevent this. I can't remember right now what is called a certain branded round cut with needle like lgf (perhaps 90% or more!) - they looked pretty weird and such exaggeration seems to defeat the initial purpose of breaking up flat pavilion facets to allow more scintillation.
Trying to sum up the ramble: before the new GOG site comes up
JohnQuixote
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After looking at thousands of these I can tell you that diamonds cut in the span of lgf being discussed can all be of top beauty with great performance through a broad range of lighting.
There are 2 things that take precedence over the minor facets: The pavilion and crown angles (in that order of importance) and the integrity of the patterning in the diamond, meaning the alignment of the mirrors in the pavilion with each other and with the crown facets. You can get an idea of patterning integrity with an ideal-scope or other reflector photo. Note that on pricescope most reflector photos you see are of those of well-patterned diamonds. Pavilion and crown angle preferences are just that: Preferences.
Strm, I disagree that 'anyone' would notice the difs between 75, 78 and 81 lgf if all 3 diamonds were cut at cherry PA/CA with good patterning. In a DiamCalc illustration yes, but in a live situation unaided by charts I think you may be overstating the differences to the casual observer. If the viewer was told what to look for I think the difs between 75 and 81 could be picked out, but it's far easier to see these differences on paper or in highly magnified static photos than it is with tiny, live, dynamic objects on your fingers.
JohnQuixote
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If you are just going to wear the diamond it doesn''t. I have seen beautiful diamonds in all of the lgf ranges being discussed.Date: 10/31/2005 9:18:01 AM
Author: valeria101
Trying to sum up the ramble: before the new GOG site comes up, I haven''t found much to look at about combinations of minors optimized for given crown and pavilion angles. How much does it matter?
Now, if you are planning to wear a thumb drive with a helium scan, computer simulations, reflector photos and a bunch of posts from PS to support your decision to snag those LGFs at a 78 avg rather than 80, maybe it does matter.
Regular Guy
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Hey, John, I''m writing because I like hearing all manner of reassuring talk. Also, however, I like it when experts can agree on the finer points, and I can stay reassured.
I read here from Bruce Harding, when he says: "note that pavilion ''mains'' are no longer dominant; the ''halves'', when 80% deep, become the dominant feature of the pavilion and are at about 42°".....that crown & pavilion angles may not tell even the bigger half of the story. Then again, I find I''m not very technical on these things, and didn''t exactly feel cleared up by my follow up.
What say you? Can you help me understand this point of view? Is it counter to your proposal, or a special case being discussed, as you see it.
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Lower girdle facets are extremely important in light return, the length of lower girdle facets makes a distinctive difference with all other factors remaining equal, but taken separately lgf-length basically means nothing.
This may sound confusing, so let me rephrase and try to clarify it.
Lower girdle facets cover the majority of the pavilion surface of a round brilliant, and therefore, they are way more important reflectors as the main pavilion facets.
The lenght of the lgf depends on the difference in angle between the lgf and the main facets. The smaller the difference, the longer the lgf.
How one judges this length, is first and foremost a matter of taste. Shorter lgf means bigger main facets, and if all is cut to extremely good angles, a broader, thicker flash and less virtual facets. Longer lgf means smaller main facets, and this will probably lead to less scintillation. Lgf somewhere in between will probably lead to a more even result, with more virtual facets, with still high flash. All in all, it is a matter of taste.
But if one looks at lgf-length separately, this means nothing. Take the case in which lgf''s are cut first, and let us cut them at 41.8°. If we cut the main facets afterwards at 40.7°, this will result in lgf-length of 78% (GIA-style of measuring). If we cut the mains at 40.5°, the lgf-length will be considerably lower (some PS-amateurs can probably calculate this); I suppose that we will quickly come to around 75%. A higher main angle, say 41.0° will result in lgf-length clearly above 80%. In these three examples, and basically in all rounds, the main contributor to light return is the angle of the lower girdle facets. As you can see, the lgf-length is only a result of the difference with the main angle.
Live long,
This may sound confusing, so let me rephrase and try to clarify it.
Lower girdle facets cover the majority of the pavilion surface of a round brilliant, and therefore, they are way more important reflectors as the main pavilion facets.
The lenght of the lgf depends on the difference in angle between the lgf and the main facets. The smaller the difference, the longer the lgf.
How one judges this length, is first and foremost a matter of taste. Shorter lgf means bigger main facets, and if all is cut to extremely good angles, a broader, thicker flash and less virtual facets. Longer lgf means smaller main facets, and this will probably lead to less scintillation. Lgf somewhere in between will probably lead to a more even result, with more virtual facets, with still high flash. All in all, it is a matter of taste.
But if one looks at lgf-length separately, this means nothing. Take the case in which lgf''s are cut first, and let us cut them at 41.8°. If we cut the main facets afterwards at 40.7°, this will result in lgf-length of 78% (GIA-style of measuring). If we cut the mains at 40.5°, the lgf-length will be considerably lower (some PS-amateurs can probably calculate this); I suppose that we will quickly come to around 75%. A higher main angle, say 41.0° will result in lgf-length clearly above 80%. In these three examples, and basically in all rounds, the main contributor to light return is the angle of the lower girdle facets. As you can see, the lgf-length is only a result of the difference with the main angle.
Live long,
JohnQuixote
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Bruce's observations are in-line with the way other cutters consider LGF length. He is saying that LGF over 80% (GIA), in his experience, become the main pavilion feature at the expense of the mains and at a reduction in contrast.
Brian and I are in agreement with the majority of what Bruce has to say here, with the exception of the restaurant example. Bruce is a colored gemstone faceter, which means he is well-versed in methods of entrapping body color. I don't pretend to be a cutter, but Brian is one of the best. In my observation (I will ask him to verify) the diamonds that react best in low-light conditions are those with pavilion angles between 40.5-40.9 and top patterning - and a lgf% range less than 80%.
strmrdr
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strmrdr
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my bottom line is according to my eyes:
edit:
imho this holds until you get to the extremes like fics then the c/p have a greater effect on the personality than the lgf but for diamonds in the super-ideal range it applies.
end edit:
c/p angles determin performance and a lot of times a lesser extent the personality
lgf and stars fine tune the personality largly and performance less so.
a tight 34.1/41 diamond is going to be bright no matter what the lgf% is.
The lgf fine tunes that light return.
I dunno John I can see the difference while they are still in the display case and Iv got bad eyeslight so I suspect that pretty much anyone could once they knew what to look for.
lgf% is one of the things that makes up the personality of a diamond that makes one diamond speak to someone over another.
I think its a fairly big part of it.
There are lighting conditions where it wont make one bit of difference but there are others that it will.
Take candlelight,
8* or 8* type will blow the others rounds away but will get blasted by my fav asschers. :}
edit:
imho this holds until you get to the extremes like fics then the c/p have a greater effect on the personality than the lgf but for diamonds in the super-ideal range it applies.
end edit:
c/p angles determin performance and a lot of times a lesser extent the personality
lgf and stars fine tune the personality largly and performance less so.
a tight 34.1/41 diamond is going to be bright no matter what the lgf% is.
The lgf fine tunes that light return.
I dunno John I can see the difference while they are still in the display case and Iv got bad eyeslight so I suspect that pretty much anyone could once they knew what to look for.
lgf% is one of the things that makes up the personality of a diamond that makes one diamond speak to someone over another.
I think its a fairly big part of it.
There are lighting conditions where it wont make one bit of difference but there are others that it will.
Take candlelight,
8* or 8* type will blow the others rounds away but will get blasted by my fav asschers. :}
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For what that matters, I would interpret the two apparently competing statements highlighted in blue above by throwing in size and color factors.... Perhaps a 6mm object needs to be (close to) a reflector in order to be seen from some distance. Perhaps at some point there is too much of a good thing though.
There is a freakish diamond cut with lgf 100% if anyone cares to see what that might look like. I found the cut disappointing in larger stones, but there are hardly any to see, so this may have been the intention of the design anyway.
Of course, 'too much of a good thing' may happen at less extreme numbers - be it 80% or above for the respective facets.
I wonder if the 'dynamic contrast' mentioned is a different matter for color and colorless: lower RI makes them unavoidable and there is color to show in a non-brilliant patch. Perhaps the excuse of 'contrast' for more than just a little leakage on colorless background holds less well, so a brighter cut fares better. The discount for colorless versions of colored precious stones may just amount to a very long-term public opinion poll.
This thread got to be quite a good read, although I also agree with your other post that words can be too much a 'magnifying glass'. Perhaps some of the content should make it up the page into the data bank of this forum (Faq?)
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