Experts opinions needed - Is this a yield issue?

[Stephan]

Why is it so difficult to find diamonds with star length of 60-65%?
If a diamond is cut with such star facets, does it loose more weight than if it's cut with 50-55% stars?
I know the formula is here:
http://www.wtocd.be/Diamond/D_Articles/The%20Brilliant%20Cut.pdf,
but I'm already sweating after calculating a lower girdle angle when knowing the lower girdle length and the pavilion angle.

If it's only a yield issue, it's a pity.
Most diamonds with longer stars that I saw had incredible fire all around.
Like the big 7.52ct ACA that Brian showed me in Houston (funny, no Sarin or new AGS certificate on this one).
http://www.whiteflash.com/hearts_arrows/A-Cut-Above-H-A-cut-diamond-72997.htm

I'm still convinced that "ideal" diamond cutters should slightly reduce the crown angle and increase the star length that are usual in the market.
Brian's big diamond is a perfect example of a slightly shallower crown with longer star facets.
Even if C34.9/P40.9 aren't "ugly", I really believe that C34/P40.8 with longer stars will look much better.

So is this a yield issue???

 

[diagem]

Date: 8/28/2008 5:24:15 PM
Author:QueenMum

Why is it so difficult to find diamonds with star length of 60-65%?
If a diamond is cut with such star facets, does it loose more weight than if it''s cut with 50-55% stars?

So is this a yield issue???

I believe its just easier to aim to the center (or in the center area) of the crown facet..., and I also assume thats one of the reasons GIA round off the numbers..., do they??

 

[JulieN]

they round it to the nearest 5. 50, 55, 60

 

[strmrdr]

Date: 8/28/2008 5:24:15 PM
Author:QueenMum

Even if C34.9/P40.9 aren't 'ugly', I really believe that C34/P40.8 with longer stars will look much better.


So is this a yield issue???

34/40.8-41 has slightly better yield but they are a bit fussy about how they are cut.
There is much more leeway with other combos.
I too like that combo with long stars and longer lgf%.
Its just another way of cutting beautiful diamonds when both are well cut with a slightly different personality than 34.9/P40.9.

 

[diagem]

Date: 8/28/2008 5:44:04 PM
Author: DiaGem

Date: 8/28/2008 5:24:15 PM
Author:QueenMum

Why is it so difficult to find diamonds with star length of 60-65%?
If a diamond is cut with such star facets, does it loose more weight than if it''s cut with 50-55% stars?

So is this a yield issue???

I believe its just easier to aim to the center (or in the center area) of the crown facet..., and I also assume thats one of the reasons GIA round off the numbers..., do they??

And to add..., no, I dont think its a yield issue at all...

 

[strmrdr]

Date: 8/28/2008 6:02:38 PM
Author: DiaGem
And to add..., no, I don''t think its a yield issue at all...

For a lot of cutters its being able to miss a bit and still get the grade.
In Brian''s case it isn''t his main preference for his idea of visual balance which is why he picked a different combo for most of his goods.

 

[Stephan]

Thank you DiaGem and Karl for your interesting answers.

Date: 8/28/2008 6:07:38 PM
Author: strmrdr

In Brian''s case it isn''t his main preference for his idea of visual balance which is why he picked a different combo for most of his goods.

So which are the differences in visual balance?

 

[diagem]

Date: 8/28/2008 6:07:38 PM
Author: strmrdr

Date: 8/28/2008 6:02:38 PM
Author: DiaGem
And to add..., no, I don''t think its a yield issue at all...

For a lot of cutters its being able to miss a bit and still get the grade.
In Brian''s case it isn''t his main preference for his idea of visual balance which is why he picked a different combo for most of his goods.

I think you are talking about the CA combination while I am talking about 55 vs 65 degree star facets...

 

[strmrdr]

Date: 8/28/2008 6:17:20 PM
Author: DiaGem
I think you are talking about the CA combination while I am talking about 55 vs 65 degree star facets...

I was but....
They actually go together.
With a shallower crown you lengthen out the stars which makes the upper girdles steeper which gives you more dispersion.
Making them steeper is also why you don't combine long stars with steeper crown angles because they become to steep and start to leak.

 

[diagem]

Date: 8/28/2008 6:30:00 PM
Author: strmrdr

Date: 8/28/2008 6:17:20 PM
Author: DiaGem
I think you are talking about the CA combination while I am talking about 55 vs 65 degree star facets...

I was but....
They actually go together.
With a shallower crown you lengthen out the stars which makes the upper girdles steeper which gives you more dispersion.
Which is also why you don''t combine long stars with steeper crown angles because they become to steep and start to leak.

You realy believe there is such a significant difference?

How much do you think the star angle would differ between a 55 and 65 deg. on a steep 35-35.5 deg. crown main?

 

[strmrdr]

disclaimer: the stars are usually cut last so long stars are the effect of steeper upper girdles.

But when designing and thinking about the effect on the crown it is easier to think of the longer stars as the cause of the steeper uppers.

 

[Stephan]

Date: 8/28/2008 6:38:34 PM
Author: DiaGem

How much do you think the star angle would differ between a 55 and 65 deg. on a steep 35-35.5 deg. crown main?

55% table, 35.5 crown with 55% stars =""> 42.68 upper girdle
same with 65% stars =""> 44.81 upper girdle

 

[strmrdr]

te:[/b] 8/28/2008 6:38:34 PM
Author: DiaGem

You realy believe there is such a significant difference?


How much do you think the star angle would differ between a 55 and 65 deg. on a steep 35-35.5 deg. crown main?[/quote]

yes 35.5CA 40.75PA 50% stars in aset

 

[strmrdr]

same combo with 65% stars.
Less red and more black and a lot of mixed shades near the edge.
You also get more reflection in the center.

 

[diagem]

Date: 8/28/2008 6:48:00 PM
Author: strmrdr
disclaimer: the stars are usually cut last so long stars are the effect of steeper upper girdles.

But when designing and thinking about the effect on the crown it is easier to think of the longer stars as the cause of the steeper uppers.

Actually..., based on my knowledge and experience..., the star facets are the first stage of the brillianteering!
And you can control the upper halves angles based on the amount of digging you are planning into the girdle plane...

 

[diagem]

Date: 8/28/2008 6:53:31 PM
Author: strmrdr
same combo with 65% stars.
Less red and more black and a lot of mixed shades near the edge.
You also get more reflection in the center.

The mixed shades near the edge depends on the angle of the halves you dig...
Notice there is no major change near the meeting point of the main with the girdle..., the change is more noticeable towards the 'digging' part of the halves (e.g. orange).

I just dont understand how the 'blue' in the center pavilion (between the pav mains) view occurs..., doesnt make sense to me..., maybe you (Garry or Paul) can elaborate more on whats causing the blue?

 

[strmrdr]

Date: 8/29/2008 7:25:39 AM
Author: DiaGem

Date: 8/28/2008 6:53:31 PM

Author: strmrdr

same combo with 65% stars.

Less red and more black and a lot of mixed shades near the edge.

You also get more reflection in the center.

The mixed shades near the edge depends on the angle of the halves you dig...

Notice there is no major change near the meeting point of the main with the girdle..., the change is more noticeable towards the 'digging' part of the halves (e.g. orange).


I just dont understand how the 'blue' in the center pavilion (between the pav mains) view occurs..., doesnt make sense to me..., maybe you (Garry or Paul) can elaborate more on whats causing the blue?

Neither combo has painting or digging it is the angle of the upper girdles in relation to the pavilion that makes the difference.
Think of it as a to steep c1 angle in a step cut.
In a round you have 2 angle relationships that change with the upper girdles getting shorter and steeper both up and down and left to right plus it changes the shape of the bezel facets.

Here are the virtual facets with 50% stars

 

[strmrdr]

With 65% stars

 

[strmrdr]

lets go with a 37 degree crown and shorter lgf% which shows the difference even more.

 

[strmrdr]

Here are the same VF images from above side by side.

 

[strmrdr]

Date: 8/29/2008 7:25:39 AM
Author: DiaGem


I just dont understand how the 'blue' in the center pavilion (between the pav mains) view occurs..., doesnt make sense to me..., maybe you (Garry or Paul) can elaborate more on whats causing the blue?

Those virtual facets are created by the interaction of the table, girdle and upper girdle facets.
With the longer stars the combo shows obstruction in highly obstructive environments due to the combination of angles and placement.
I rarely see them in person they are more of an issue under the ASET than in real life.

 

[diagem]

Date: 8/29/2008 10:33:18 AM
Author: strmrdr

Date: 8/29/2008 7:25:39 AM
Author: DiaGem

I just dont understand how the ''blue'' in the center pavilion (between the pav mains) view occurs..., doesnt make sense to me..., maybe you (Garry or Paul) can elaborate more on whats causing the blue?


So I understand the blue is a reflection of a bit more obstruction/head shadow as the star facets become a bit steeper...

Neither combo has painting or digging it is the angle of the upper girdles in relation to the pavilion that makes the difference.
Think of it as a to steep c1 angle in a step cut.
In a round you have 2 angle relationships that change with the upper girdles getting shorter and steeper both up and down and left to right plus it changes the shape of the bezel facets.

Here are the virtual facets with 50% stars

All upper girdle facets are dug..., some are more ("digging") and some are less ("painting")....
But I still believe the cutter can control the ugf''s with no connection to the size of the star facet...

The problem is that most cutters dont know what to control for what effects?

 

[strmrdr]

Date: 8/29/2008 11:48:39 AM
Author: DiaGem
All upper girdle facets are dug..., some are more ('digging') and some are less ('painting')....

But I still believe the cutter can control the ugf's with no connection to the size of the star facet...


The problem is that most cutters dont know what to control for what effects?

You are making a simple matter complicated.
Of course the cutter can dig or paint the uppers instead of cutting them to a classic profile but it is not the same thing as lengthening the stars and changing the angle and position relationships.

 

[diagem]

Date: 8/29/2008 11:58:56 AM
Author: strmrdr

Date: 8/29/2008 11:48:39 AM
Author: DiaGem
All upper girdle facets are dug..., some are more (''digging'') and some are less (''painting'')....

But I still believe the cutter can control the ugf''s with no connection to the size of the star facet...


The problem is that most cutters dont know what to control for what effects?

You are making a simple matter complicated.
Of course the cutter can dig or paint the uppers instead of cutting them to a classic profile but it is not the same thing as lengthening the stars and changing the angle and position relationships.

What is the "classic profile"?

 

[strmrdr]

Date: 8/29/2008 12:01:33 PM
Author: DiaGem

What is the 'classic profile'?

http://lgdl.gia.edu/pdfs/estimating_painitng.pdf

Classic would be what GIA calls negligible on page 3 and 4 of the pdf file.

 

[diagem]

Thanks...

I think the one with the longer star facets does have an interesting "something" to it....

 

[Stephan]

Wow, thank you so much for all the pictures, Karl.

Date: 8/28/2008 6:02:36 PM
Author: strmrdr

I too like that combo with long stars and longer lgf%.
Its just another way of cutting beautiful diamonds when both are well cut with a slightly different personality than 34.9/P40.9.

Karl, can you tell me what kind of difference you see?

 

[Stephan]

GoodOldGold video
Even if the diamond on the left looks better than the one on the right to my eye, I guess it would even look more firey with a 40.8 pavilion.
Do you agree?

 

[strmrdr]

Date: 8/29/2008 3:59:16 PM
Author: QueenMum
GoodOldGold video

Even if the diamond on the left looks better than the one on the right to my eye, I guess it would even look more firey with a 40.8 pavilion.

Do you agree?

No I don''t agree on the 40.8 pavilion, its not that simple.
Crown angle and lgf% has more to do with if a diamond has a difference in fire from another than pavilion angle when the pavilion angle is matched to the crown angle. If 40.8 and 41 are both well matched to the crown with the same lgf% there is little if any visible difference going from one to the other.
Then the angle of the upper girdles and corresponding stars have a modifying effect from there.

What you run into is that like a lot of things with diamonds is that the upper girdle angles and corresponding stars will go from bad to good fast then stay good for a fairly wide range then drop off fast again.

Which is why GIA and many cutters don''t consider stars that important as long as they can hit the good range without any worry.
Which is why you see a lot at 50%-55% stars and classic girdles as they work well with a lot of combos even if they aren''t always the best.

 

[diagem]

Date: 8/29/2008 5:01:59 PM
Author: strmrdr

Date: 8/29/2008 3:59:16 PM
Author: QueenMum
GoodOldGold video

Even if the diamond on the left looks better than the one on the right to my eye, I guess it would even look more firey with a 40.8 pavilion.

Do you agree?

No I don''t agree on the 40.8 pavilion, its not that simple.
Crown angle and lgf% has more to do with if a diamond has a difference in fire from another than pavilion angle when the pavilion angle is matched to the crown angle. If 40.8 and 41 are both well matched to the crown with the same lgf% there is little if any visible difference going from one to the other.
Then the angle of the upper girdles and corresponding stars have a modifying effect from there.

What you run into is that like a lot of things with diamonds is that the upper girdle angles and corresponding stars will go from bad to good fast then stay good for a fairly wide range then drop off fast again.

Which is why GIA and many cutters don''t consider stars that important as long as they can hit the good range without any worry.
Which is why you see a lot at 50%-55% stars and classic girdles as they work well with a lot of combos even if they aren''t always the best.

Couldnt have said it better..., its not that cutters dont consider stars that important..., they dont know its supposed to be important...