Do large Stars and small Lower Halves decrease brilliance?

[Daniel S.]

Do large Stars and small Lower Halves decrease a Round Brilliant cut diamond's brilliance? While attempting to learn more about H&A cut stones I've noticed certain characteristics: Large stars create large "hot spots" (terminology obtained from GOG's minor facet guide, is there a proper name?) surrounding the center arrow shafts, and small lower halves create very large arrows in the diamond. Both of these characteristics appear to eliminate quite a bit of the white space when looking into the table of the diamond. I have attached some images to describe what I'm referring too kindly taken from GOG's web diamond emporium (I hope this is ok with forum administrators, the logo has been chopped off of each image).

This first image is a diamond with 55% stars. The "hot spots" created by the larger stars seem to eliminate quite a bit of the white space within the table.


This second image is a diamond with 75% lower halves. The very large arrow shafts seem to eliminate quite a bit of the white space within the table.



Additionally, the difference that the lower halves make in the heart images seem very clear in these two examples. Could these both be classified as the upper and lower extent of the accepted range of Hearts and Arrows or could a Cutter push the envelope slightly further and still land within the threshold? (By the way, is this threshold vendor determined?) Also, are the lower halves solely responsible for the image characteristics I describe with each image?

This first image shows a diamond with 80% lower halves. The higher lower halves seem to be responsible for the large distance between the heart and arrow (is that it's proper name), the cut cleft, and the not sharply defined tips of the arrows adjacent to the base of the heart.



This second image shows a diamond with 75% lower halves. The smaller lower halves seem to be responsible for the nearly not cut clefts (although seem appear cut?), the very small distance between the arrow valley and the base of the heart, and the seemingly much greater convergence of the arrow tips at the center of the stone as compared to the example diamond above.



Next, there has been a lot of discussion lately about crispness of H&A patterns. Could any of the very defining characteristics of the two H&A images above be due to not cutting the H&A image in manner that creates crispness? I realize that some of the roughness around the arrow tips above the arrow valley may be due to photography.

Finally, my Wonderful Fiancé and I were working with our wedding coordinator and we both noticed that her diamond ring had seemingly no contrast when looking in the table. The diamond seemed to well cut but no matter how close I looked (while trying to be completely natural of course ) I could not once see the black arrow shafts. Is there some cut characteristic that would cause this or is it that I never got close enough for my head to darken her diamond's arrow shafts?

Thanks PS for this wonderful education!

 

[stone-cold11]

Re: Do large Stars and small Lower Halves decrease brillianc

Please look at your definition of table again. What you are describing is not making sense as the stars are facets outside the table facet and will not have any effect in the virtual facets within the table.

Stars and lower half are described as a proportion of the main crown and pavilion facets, by themselves alone, they are meaningless.
Stars generally has more leeway as they are not that important to light return, more as a aesthetic purpose. Lower half are important for light return, leakage generally occurs in lower half facets. Arbitrarily setting lower half size to 75% while ignoring the overall crown and pavilion angles will lead to bad performing H&A, with light leakage under the table where the lower half facets are.

For the wedding coordinator's stone, depends on the lighting condition when you are viewing the stone, how close are you looking at it? Less than 1 foot away?

 

[Karl_K]

Re: Do large Stars and small Lower Halves decrease brillianc

As Stone-cold11 said you can not separate out one set of facets then make general statements.
It all depends on the rest of the angles and %.
Every facet counts.
As far as the lowers go start here:
https://www.pricescope.com/journal/do_pavilion_mains_drive_light_return_modern_round_brilliant

 

[WinkHPD]

Re: Do large Stars and small Lower Halves decrease brillianc

Snip

The diamond seemed to well cut but no matter how close I looked (while trying to be completely natural of course ) I could not once see the black arrow shafts. Is there some cut characteristic that would cause this or is it that I never got close enough for my head to darken her diamond's arrow shafts?

Thanks PS for this wonderful education!

Normally it is difficult to see the obstruction pattern when looking at a diamond in normal lighting. You might try cupping your hand (like making an imaginary telescope the way we did as kids) and holding the ring at the far end of the cup. This will often allow you to see the obstruction pattern.

Wink

P.S. This will ruin the attempt to appear natural, of course.

(Edited to add PS.)

 

[Andelain]

Re: Do large Stars and small Lower Halves decrease brillianc

Tagging this to find later.

 

[yssie]

Re: Do large Stars and small Lower Halves decrease brillianc

A)
depth62.3/table57 35.1crown/pav40.7
star62/lgf79
http://www.whiteflash.com/loose-diamond ... 219628.htm

B)
60.5/55 32.9/40.9
53/82
http://www.whiteflash.com/loose-diamond ... 874238.htm

C)
60.2/57 34.4/40.8
57/76
http://www.whiteflash.com/loose-diamond ... 192350.htm

D)
61.3/55.5 34.7/40.8
54/76
http://www.whiteflash.com/loose-diamond ... 141810.htm

 

[Daniel S.]

Re: Do large Stars and small Lower Halves decrease brillianc

Please look at your definition of table again. What you are describing is not making sense as the stars are facets outside the table facet and will not have any effect in the virtual facets within the table.

Thank you Stone-cold11. I re-reviewed the GOG's minor facet page and my mistake was in thinking that the stars are the direct determinants of "hot-spots" surrounding the arrow shafts under the table. According to that same guide, it appears that these spots are more directed related to lower half sizes.

Stars and lower half are described as a proportion of the main crown and pavilion facets, by themselves alone, they are meaningless. Stars generally has more leeway as they are not that important to light return, more as a aesthetic purpose. Lower half are important for light return, leakage generally occurs in lower half facets. Arbitrarily setting lower half size to 75% while ignoring the overall crown and pavilion angles will lead to bad performing H&A, with light leakage under the table where the lower half facets are.

I think this is where my confusion stems. In the process of cutting or polishing a diamond (when is the transition?) are the crown and pavilion angles generally arrived at considering the rough and the desired amount of retained weight, the crown and the pavilion are cut, and then the star and lower half sizes are cut to create a good H&A image?

As Stone-cold11 said you can not separate out one set of facets then make general statements.
It all depends on the rest of the angles and %.

I have heard this before. It seems that some values i.e., depth, table, crown and pavilion angle, allow you to make valid comparisons between stones within reason, or in essence, separate out a facet(s). For the purposes of comparing diamonds unseen, it would be great if there was a good way to make a valid comparison based on what sort effects the minor facets will have on performance. Maybe because we can’t we rely on ideal-scopes or ASET images?

A)
depth62.3/table57 35.1crown/pav40.7
star62/lgf79
http://www.whiteflash.com/loose-diamond ... 219628.htm

….

Yssie, I am assuming you kindly went through all this trouble of finding these four stones to show me that a wide variety of crown, pavilion, lower half, and star values can produce very similar results?

 

[stone-cold11]

Re: Do large Stars and small Lower Halves decrease brillianc

The table, crown and pavilion angles, with girdle thickness determines the outline of the stone. Minor facets are cut into these outline. Polishing stage, can be cut both ways, pavilion mains first then lower half or vice versa.

I have heard this before. It seems that some values i.e., depth, table, crown and pavilion angle, allow you to make valid comparisons between stones within reason, or in essence, separate out a facet(s). For the purposes of comparing diamonds unseen, it would be great if there was a good way to make a valid comparison based on what sort effects the minor facets will have on performance. Maybe because we can’t we rely on ideal-scopes or ASET images?

And you think it is better to rely on numbers on a grading report that are averaged and then rounded? Do not understand the logic there.

 

[Karl_K]

Re: Do large Stars and small Lower Halves decrease brillianc

Trade members are not going to be allowed to comment on the WF stones above.

 

[Stephan]

Re: Do large Stars and small Lower Halves decrease brillianc

Please look at your definition of table again. What you are describing is not making sense as the stars are facets outside the table facet and will not have any effect in the virtual facets within the table.

I don't think that this statement is correct.

 

[Daniel S.]

Re: Do large Stars and small Lower Halves decrease brillianc

The table, crown and pavilion angles, with girdle thickness determines the outline of the stone. Minor facets are cut into these outline. Polishing stage, can be cut both ways, pavilion mains first then lower half or vice versa.

Thanks!

I have heard this before. It seems that some values i.e., depth, table, crown and pavilion angle, allow you to make valid comparisons between stones within reason, or in essence, separate out a facet(s). For the purposes of comparing diamonds unseen, it would be great if there was a good way to make a valid comparison based on what sort effects the minor facets will have on performance. Maybe because we can’t we rely on ideal-scopes or ASET images?

And you think it is better to rely on numbers on a grading report that are averaged and then rounded? Do not understand the logic there.

The question arises for a few reasons.

One, certain vendors single out a specific facet and describe the effect on the diamond's appearance and performance caused by varying this specific facet. When asking for further clarification often the answer is "you can not single out a specific facet for investigation" My hope is to understand at least the general consensus on the effect of these variations.

Two, when looking to purchase a diamond online, a ASET image or Idealscope image are not always available or provided. The effect that table, crown, pavilion, and girdle have on overall appearance seems fairly clear in most cases. The effect that different star and lower half sizes (inconjunction with the other facets) have is not always very clear.

Although these values may be rounded, I see some value in obtaining a better understanding of their contribution to overall diamond appearance and performance.

 

[stone-cold11]

Re: Do large Stars and small Lower Halves decrease brillianc

The question arises for a few reasons.

One, certain vendors single out a specific facet and describe the effect on the diamond's appearance and performance caused by varying this specific facet. When asking for further clarification often the answer is "you can not single out a specific facet for investigation" My hope is to understand at least the general consensus on the effect of these variations.

Two, when looking to purchase a diamond online, a ASET image or Idealscope image are not always available or provided. The effect that table, crown, pavilion, and girdle have on overall appearance seems fairly clear in most cases. The effect that different star and lower half sizes (inconjunction with the other facets) have is not always very clear.

Although these values may be rounded, I see some value in obtaining a better understanding of their contribution to overall diamond appearance and performance.

Think here for a bit and you will understand why. What does a larger lower half size implies for the angle of the lower half facet?

Making it closer to the main pavilion angle.

So to prevent leakage, assuming the main pavilion is not cut at an angle that is so big that even it is leaking light, you want that angle of the lower half facet to be closer to that of the main pavilion, so larger lower half size. But cut too big, the main pavilion facet, arrows, becomes skinny, some do not like that. Also, slightly more weight is loss with a larger lower half, but generally negligible, less than 1%, unless in the larger carat sizes.

This means, if at the boundary where, HCA is around 2-3, leakage start showing, there can be cases where expected leakage does not occur due to larger lower half sizes. Assuming good symm. So one cannot really tell with certainty if there is leakage or not based on averaged and rounded numbers, only maybe can work, if lucky.

The main pavilion is also in the obstruction range, if it is in the range where obstruction might be an issue, around 40.6 and below. So, cut a larger lower half, less of the main pavilion is obstructing, but too much, the lower half starts obstructing too.

So, again, maybe can work, if lucky, based on numbers.

So back to IS/ASET image to see if these numbers work together. Else, basing on numbers is a shot in the dark.

Compare the HCA scores of these 2 stones, lower half sizes and then their IS images.

http://www.whiteflash.com/loose-diamonds/round-cut-loose-diamond-2192329.htm

http://www.jamesallen.com/diamonds/H-SI1-Ideal-Cut-Round-Diamond-1297361.asp?b=16&a=12&c=77&cid=131

 

[stone-cold11]

Re: Do large Stars and small Lower Halves decrease brillianc

Please look at your definition of table again. What you are describing is not making sense as the stars are facets outside the table facet and will not have any effect in the virtual facets within the table.

I don't think that this statement is correct.

Point to me, from the above GOG diamond images, where under the table facets, is a patterning cause by the star facets.

 

[Stephan]

Re: Do large Stars and small Lower Halves decrease brillianc

not only the star facet

 

[yssie]

Re: Do large Stars and small Lower Halves decrease brillianc

The question arises for a few reasons.

One, certain vendors single out a specific facet and describe the effect on the diamond's appearance and performance caused by varying this specific facet. When asking for further clarification often the answer is "you can not single out a specific facet for investigation" My hope is to understand at least the general consensus on the effect of these variations.

Two, when looking to purchase a diamond online, a ASET image or Idealscope image are not always available or provided. The effect that table, crown, pavilion, and girdle have on overall appearance seems fairly clear in most cases. The effect that different star and lower half sizes (inconjunction with the other facets) have is not always very clear.

Although these values may be rounded, I see some value in obtaining a better understanding of their contribution to overall diamond appearance and performance.

SC posted a fantastic writeup

My 2c - I have seen this done too, more often incompletely than incorrectly - as in, a distinction will be made that implies a cascade of effects that aren't stated. The first example that comes to mind is "larger table will make a stone look larger".

Well - that's not the whole story, is it? If table glare makes the stone look larger, that's a directly causal relationship. There is also the fact that larger table often means shallow crown, which when combined w/ a shallower pavilion does indicate larger spread than a tolk-type - the volume must go somewhere! But there is a series of requirements there that if not fulfilled voids the relationship.

The WF examples were to show that the black triangles at arrow heads/shafts exist depending on long lgf, and can be seen when star is both long and short, and they are only as visibly pronounced as they actually exist, no matter what star length