Do the pavilion mains drive light return in the modern round brilliant?

[psadmin]

New Article by Karl K ( aka Strmrdr )

Do the pavilion mains drive light return in the modern round brilliant?


Thank you Karl for your contribution.

 

[strmrdr]

Thank you Andrey for the opportunity to present this article.

I have been asked many times why I ask for the lgf% when the pavilion angle approaches or exceeds 41 degrees with a crown angle over 34.5 degrees.
This is one of the biggest reason.
This is also one reason to always ask for images as the numbers do not tell the whole story as the lgf% average may not reflect the actual range of angles in the lower girdles.
At times you will find diamonds that some of the lower girdle facets are over and some are under the angle where leakage begins for that particular combination

That is the easy part now it gets hard.

The other reason is that some combinations work better with different lgf% in regards to the distribution of the light return even where there is no leakage present.
This comes into play particularly with 41 degree pavilions and 33-34.5 crown angles with 55-57 tables.
Diamonds with 40.7-40.8 pavilion and crown angles in the 34-35 range it is less critical as they work with a wider range of lgf%
That is a much harder subject to present in an understandable manner but I am working on it.

The third case is shallow pavilions where obstruction becomes an issue and a higher lgf% means narrower arrows and obstruction is less of an issue.(fic and bic cuts)
In the future I hope to present articles on these subjects as I find ways to explain them in an easy to understand manner.

 

[Lorelei]

Karl, its an excellent article, well written and easy to understand - if I may quote one of your sayings...CONGRATES!!!

 

[Regular Guy]

Thanks, Karl, I think this is a good article, but I'm a little thick about some things. Remove some of the technical underpinnings, and it probably becomes too long.

One big thing you said I'm betting, but I want to confirm...cause you didn't say it out right...

the Morse Style diamond has a quite small crown...right?

This will affect how the crown angle meets the girdle, and impacts the preferred angle of where the pavilion angle meets the girdle, right?

Separately, and your main point, I think...is that the minor facets, in this latter case, then become important...or maybe critically important in understanding performance.

I'll read it again and this will help, I'm sure.

But did what I review here make sense, too?

Many thanks.

John P was right, though....you could have been flying turbo jets by now.

 

[strmrdr]

Date: 3/12/2009 5:56:44 PM
Author: Regular Guy
Thanks, Karl, I think this is a good article, but I'm a little thick about some things. Remove some of the technical underpinnings, and it probably becomes too long.


One big thing you said I'm betting, but I want to confirm...cause you didn't say it out right...


the Morse Style diamond has a quite small crown...right?
small table yes, the crown is tall but narrow, see image

This will affect how the crown angle meets the girdle, and impacts the preferred angle of where the pavilion angle meets the girdle, right?
not sure what you are saying here

Separately, and your main point, I think...is that the minor facets, in this latter case, then become important...or maybe critically important in understanding performance.
correct the lower girdles become critical in a modern RB


I'll read it again and this will help, I'm sure.


But did what I review here make sense, too?


Many thanks.


John P was right, though....you could have been flying turbo jets by now.

 

[oldmancoyote]

Karl, thanks for the article. May I ask a question? You write:

Let’s take a modern Round Brilliant many experts would call steep deep: 41.2 degree pavilion angle and 35 degree crown angle with 80% lower girdle facet % and a 57% table.

As you can see there is extensive under table leakage mixed with obstruction.

How can the cutter remove the under-table leakage?

3 ways:

A: Remove the lower girdle facets from under the table by shortening them to 40%, image A.

B: Bring the angles of the lower girdles down by lengthening them out to 83% and an angle of 42.24 degrees. Image B.

C: Paint the lower girdles to an angle of 42.24 degrees. Image C.

Did you run any analysis as to what is the effect on retained weight of the three options?

 

[strmrdr]

Date: 3/15/2009 4:04:49 PM
Author: oldmancoyote

Did you run any analysis as to what is the effect on retained weight of the three options?

A: slightly heavier but within .01ct
B: within .00x
C: within .00x

 

[Regular Guy]

Karl,

Doing some basic fact checking here mostly...

I think your thesis may be that these days, we''ve come to think that the pavilion angle is all that because that''s how is also used to be with the Morse design, where you had a smaller table...and that perhaps, in error, we''ve over-attributed importance to that angle, when it doesn''t apply so well any more where the table is a good bit larger.

I DO see where, as you say, "The lower girdles do not extend under the table in Morse/Wade like they do in the modern round brilliant...." but....what you may already have calculated anyway, but not shown....presuming you have the results you do with Morse, at 42.34, and you have leakage... At that point, can you also modify the minor facets, and again, reduce considerably the leakage?

 

[strmrdr]

Date: 3/15/2009 5:17:48 PM
Author: Regular Guy
Karl,


Doing some basic fact checking here mostly...


I think your thesis may be that these days, we''ve come to think that the pavilion angle is all that because that''s how is also used to be with the Morse design, where you had a smaller table...and that perhaps, in error, we''ve over-attributed importance to that angle, when it doesn''t apply so well any more where the table is a good bit larger.


I DO see where, as you say, ''The lower girdles do not extend under the table in Morse/Wade like they do in the modern round brilliant....'' but....what you may already have calculated anyway, but not shown....presuming you have the results you do with Morse, at 42.34, and you have leakage... At that point, can you also modify the minor facets, and again, reduce considerably the leakage?

In Morse/Wade extending the lgf under the table reduces the possible pavilion angle to avoid leakage. (it becomes a small tabled modern RB and acts the same)
Shortening them further has no effect on under table leakage.

Changing the table size when the crown angle is the same and the lower girdles do not extend under the table does not change the table leakage angles it just changes the amount of table leakage area. (some more and some less total under table leakage area but that is whole different ball game and needs more research)

 

[Regular Guy]

Date: 3/15/2009 6:37:47 PM
Author: strmrdr

Date: 3/15/2009 5:17:48 PM
Author: Regular Guy
Karl,


Doing some basic fact checking here mostly...


I think your thesis may be that these days, we''ve come to think that the pavilion angle is all that because that''s how is also used to be with the Morse design, where you had a smaller table...and that perhaps, in error, we''ve over-attributed importance to that angle, when it doesn''t apply so well any more where the table is a good bit larger.


I DO see where, as you say, ''The lower girdles do not extend under the table in Morse/Wade like they do in the modern round brilliant....'' but....what you may already have calculated anyway, but not shown....presuming you have the results you do with Morse, at 42.34, and you have leakage... At that point, can you also modify the minor facets, and again, reduce considerably the leakage?

In Morse/Wade extending the lgf under the table reduces the possible pavilion angle to avoid leakage. (it becomes a small tabled modern RB and acts the same)
Shortening them further has no effect on under table leakage.

Changing the table size when the crown angle is the same and the lower girdles do not extend under the table does not change the table leakage angles it just changes the amount of table leakage area. (some more and some less total under table leakage area but that is whole different ball game and needs more research)

Since you can modify also the minor facets earlier, just as now, to remove leakage, does this effect your thesis that substantive attention to the pavilion angle had been appropriate, but is now misplaced?

Generally, since shoppers will wish to pay attention where it makes sense to do so, is there any reason fundamentally to do any different now than there ever was cause to attend to?

Clearly, under normal circumstances, minor changes to the pavilion angle can make a big difference...and a big enough one that grading labs have adopted this primary reference for displaying data intended to describe cut, and attendant performance. I''m not sure the extent to which the minor facets play an important place in the big scheme of things. I do think, though, that essays like yours do help to illustrate that the crown and pavilion angles do not alone an analysis make, and that more should be considered for the bigger picture...probably with the use of reflector technologies as a helpful aid.

 

[strmrdr]

Changing the table size when the crown angle is the same and the lower girdles do not extend under the table does not change the table leakage angles it just changes the amount of table leakage area.

 

[Kaleigh]

Date: 3/12/2009 6:12:47 AM
Author: Lorelei
Karl, its an excellent article, well written and easy to understand - if I may quote one of your sayings...CONGRATES!!!

Yes Lorelie says it best for me. CONGRATES my dear friend. kudos to you!!!!!!!

 

[strmrdr]

Date: 3/15/2009 6:47:58 PM
Author: Regular Guy

Since you can modify also the minor facets earlier, just as now, to remove leakage, does this effect your thesis that substantive attention to the pavilion angle had been appropriate, but is now misplaced?


Generally, since shoppers will wish to pay attention where it makes sense to do so, is there any reason fundamentally to do any different now than there ever was cause to attend to?

With the old designs with a to steep pavilion angle you can not remove the leakage by adjusting the lower girdles, you have to move the mains.
Changing the lower girdles just changes the leakage pattern.

As far as avoiding leakage when shopping goes it is the same for both, view it under an IS/ASET.

 

[strmrdr]

Date: 3/15/2009 6:47:58 PM
Author: Regular Guy

Clearly, under normal circumstances, minor changes to the pavilion angle can make a big difference...and a big enough one that grading labs have adopted this primary reference for displaying data intended to describe cut, and attendant performance. I''m not sure the extent to which the minor facets play an important place in the big scheme of things. I do think, though, that essays like yours do help to illustrate that the crown and pavilion angles do not alone an analysis make, and that more should be considered for the bigger picture...probably with the use of reflector technologies as a helpful aid.

Pavilion angles are easy it is just one number, lower girdles are hard its an angle and length.
It is a lot easier to just say hey this pavilion angle is good this one not so good with the normal range of lower girdles than consider the effect of the lowers on an individual basis.
Ray tracing/reflector based vs 2D look up charts....

Reflectors are the easy answer on finished goods but doesn''t tell you how to avoid it when cutting.

 

[strmrdr]

Date: 3/15/2009 6:54:48 PM
Author: Kaleigh

Date: 3/12/2009 6:12:47 AM

Author: Lorelei

Karl, its an excellent article, well written and easy to understand - if I may quote one of your sayings...CONGRATES!!!

Yes Lorelie says it best for me. CONGRATES my dear friend. kudos to you!!!!!!!

Thank you!!!!

Thank you Lorelei!!!

 

[oldmancoyote]

Date: 3/15/2009 4:40:35 PM
Author: strmrdr

Date: 3/15/2009 4:04:49 PM
Author: oldmancoyote

Did you run any analysis as to what is the effect on retained weight of the three options?

A: slightly heavier but within .01ct
B: within .00x
C: within .00x

Interesting - thank you. So there is no apparent reason (apart from cutting time/precision) not to go for any of the above, and - even more interesting to me - painting isn''t necessarily negative, particularly on steeply cut pavillions. Or am I generalising too much?

 

[strmrdr]

Date: 3/16/2009 10:23:18 AM
Author: oldmancoyote

Date: 3/15/2009 4:40:35 PM

Author: strmrdr

Date: 3/15/2009 4:04:49 PM

Author: oldmancoyote

Did you run any analysis as to what is the effect on retained weight of the three options?

A: slightly heavier but within .01ct

B: within .00x

C: within .00x

Interesting - thank you. So there is no apparent reason (apart from cutting time/precision) not to go for any of the above, and - even more interesting to me - painting isn''t necessarily negative, particularly on steeply cut pavillions. Or am I generalising too much?

A: would be hard to sell.
B: Could get AGS0 and GIA EX but would have clefts in the hearts so some people will scream it is not h&a.
C: would likely be able to get AGS0 but GIA EX might not be possible.

Painting and digging can be used for good or evil.
What I don''t like is painting for the sake of painting or painting for marketing(all red reflector image blah blah blah)
In my personal opinion there is absolutely no reason a 34.x/40.6-40.9/50%-57%t diamond should ever be painted to any degree(all cut diamonds have some painting and digging). Some will disagree.

 

[pyramid]

Thank you storm.

 

[strmrdr]

Date: 3/18/2009 4:08:15 PM
Author: Pyramid
Thank you storm.

Your welcome

 

[stone-cold11]

Date: 3/15/2009 7:06:36 PM
Author: strmrdr
Pavilion angles are easy it is just one number, lower girdles are hard its an angle and length.

Karl, I do not understand this statement, are you saying the LGF length and angle are independent of each other? I always thought that for a certain PA, the LGF angles and length are not independent, no?

 

[strmrdr]

Date: 3/19/2009 2:54:16 PM
Author: Stone-cold11

Date: 3/15/2009 7:06:36 PM

Author: strmrdr

Pavilion angles are easy it is just one number, lower girdles are hard its an angle and length.

Karl, I do not understand this statement, are you saying the LGF length and angle are independent of each other? I always thought that for a certain PA, the LGF angles and length are not independent, no?

painting and digging can change the angle with the same length and the same pavilion main angle.
It is actually pretty common in most diamonds for there to be some even in the best cut diamonds but it is a small amount.
In the article example C has the same lgf% as the one that leaks.

 

[risingsun]

Very informative article, strm. Thanks for adding new info to this site

 

[Lula]

Hi, Karl,
I''ve finally done enough reading/research on Pricescope to understand (loosely speaking! !) this article. I also a fan of the Morse or 35/41 combo. I am looking at a stone now that has a 34.5 crown angle and a 41.0 pavilion angle (AGS 0 grade) with 56.7 table and a 61 depth, color D; VS1; a little over 1/2 carat .512).

I have a question about something you mentioned in your article about this combination -- quoting:

"The other reason is that some combinations work better with different lgf% in regards to the distribution of the light return even where there is no leakage present.
This comes into play particularly with 41 degree pavilions and 33-34.5 crown angles with 55-57 tables."

What lgf should I be looking for in this stone? I like the look of longer girdles in a stone (79 - 80 %)
but I''m not sure if the lgf reported by AGS for this stone (81) would be problematic with the other numbers.

Please advise -- thanks!

 

[strmrdr]

Date: 4/18/2009 5:58:44 PM
Author: sarap333
Hi, Karl,

I've finally done enough reading/research on Pricescope to understand (loosely speaking! !) this article. I also a fan of the Morse or 35/41 combo. I am looking at a stone now that has a 34.5 crown angle and a 41.0 pavilion angle (AGS 0 grade) with 56.7 table and a 61 depth, color D; VS1; a little over 1/2 carat .512).


I have a question about something you mentioned in your article about this combination -- quoting:


'The other reason is that some combinations work better with different lgf% in regards to the distribution of the light return even where there is no leakage present.

This comes into play particularly with 41 degree pavilions and 33-34.5 crown angles with 55-57 tables.'


What lgf should I be looking for in this stone? I like the look of longer girdles in a stone (79 - 80 %)

but I'm not sure if the lgf reported by AGS for this stone (81) would be problematic with the other numbers.


Please advise -- thanks!

81 is perfect for that combo.
assuming no painting or digging.
Do you have an IS image?

 

[Lula]

I was hoping you''d say 81 is perfect Here''s the IS. If it checks out okay, this could be "the one."

 

[strmrdr]

Date: 4/18/2009 6:42:01 PM
Author: sarap333
I was hoping you''d say 81 is perfect Here''s the IS. If it checks out okay, this could be ''the one.''

looks fine, slight painting on the uppers but not enough to matter.
Nice diamond :}

 

[Lula]

Thanks so much for your opinion, Karl. And thanks for being an advocate for customers like me who like to push the edge of the ideal cut! I''m ordering it for a 30-day review and I''ll let you know how it checks out in real life.

 

[strmrdr]

Date: 4/18/2009 7:06:18 PM
Author: sarap333
Thanks so much for your opinion, Karl. And thanks for being an advocate for customers like me who like to push the edge of the ideal cut! I''m ordering it for a 30-day review and I''ll let you know how it checks out in real life.

kewl

 

[Skippy123]

Great article Karl; love the visuals

 

[strmrdr]

Date: 4/19/2009 9:49:39 PM
Author: Skippy123
Great article Karl; love the visuals

Thanks skip skip skippy!