Correlation between symmetry and light performance

[rogue]

We know that a top symmetry grade does not indicate the light performance, which is determined by the cut. So, for example, a very deep round stone with a 42 pavilion angle will likely not have good light performance, though it may have excellent symmetry, even hearts and arrows symmetry. Yet I can also imagine a situation where the stone is cut optimally as far as angles, but the symmetry and facet alignment is so poor that the light performance may be throw off. Are there any circumstances that would cause a stone with both 1) good angles and other cut criteria (as determined by the HCA or other accepted measurements) and 2) excellent symmetry to have poor light performance?

 

[Paul-Antwerp]

Rogue,

You are touching one of my pet peeves with your remark.

Let me start by saying that in the assessment of symmetry, I personally do not consider the mere presence of H&A to be the ultimate in symmetry. Generally, I think that the term H&A is applied too loosely, also by the Japanese labs who issue H&A-grading reports, and there are definitely different levels of H&A.

When looking at a diamond, you see various virtual facets or windows. With increased symmetry, and I mean not only an H&A-image, but also very small differences between the same angles and exact positioning of the facets, the borders of these virtual facets become more crisp. You can also see this in the crispness of the H&A-pattern, but I must say that it is very difficult to photograph. This needs to be seen in real life.

According to me, that increased crispness increases the scintillation, in the sense that it the whole stone has clearly defined borders between light and dark, and no greyish borderlines.

In the past, some consumers have done Pepsi-tests of Infinity-stones, compared to other branded H&A''s, and it is exactly this phenomenon of very crisp scintillation, which they described as being more pronounced in our stones.

In cutting princesses, I have learned even more that the importance of this level of geometrical symmetry is very important. Unfortunately, it is a very complicated subject to study.

Hope this answers your question.

 

[WinkHPD]

Last May at the "Cut Band Camp" held by Peter Yantser prior to the Las Vegas Gem show he commented to those of us there that the AGS research had shown that the Hearts and Arrows phenomenon did increase the contrast and scintillation of even the ideal cut diamonds, ie, you could have an ideal cut without the H&A pattern, but it would in most cases be even more spectacular with the H&A than without it. This may be only a small difference and hard for most people to see, but our pricescopers are a VERY demanding crowd.

Part of the AGS research shows that the eye is hard wired to pick up edges and that it is therefor a good thing for the diamond to have many edges between on/off brightness. I have included a slide from the presentation that Mr. Yantzer gave at his presentation at the Vegas show. (With permission.)

As Paul stated above, the more precise the pattern the better the appearance of the stone, even if the amount of light returned is actually the same between two stones. It has to do with how we see and determine brightness, or actually how we determine apparent brightness.

Wink

 

[WinkHPD]

Here is an example of improper contrast, at least to the human eye''s point of view...

 

[Paul-Antwerp]

Thank you for these slides, Wink. It clearly explains the concept of virtual facets.

Now, in a geometrically more symmetrical diamond, the borders of these virtual facets will be more clearly defined, more crisp, and not fuzzy. This in turn increases the visible scintillation.

Live long,

 

[oldminer]

"Are there any circumstances that would cause a stone with both 1) good angles and other cut criteria (as determined by the HCA or other accepted measurements) and 2) excellent symmetry to have poor light performance?"


Yes. This is the point that I have been attempting to press forward on in some earlier postings.
You can have what appear to be "good angles" to measuring devices that are still painted or dug out in ways that might have a pronounced effect on performance. Some of these changles are nearly imperceptble to human vision and most measuring devices, but quite dramatric at the extremes when it comes to the final look of the diamond.

A stone with excellent symmetry may have problems with light performance due to even small anglular changes or small facet length changes. Symmetry does creat better contrast/intensity, but it is not the only variable necessary for beauty. It is an important component of the situation.

Please consider the more likely occurence of minor cloudiness or UV fluorescence which also could cause a measureable change in light performance, too. Diamonds with large, or dark inclusions may have altered light perfromance in spite of their fine cut details.

In other words, it is a complex subject. Some degradation of light behavior may be measured before it becomes eye visible. This follows the typical way diamonds are graded by very fine increments and not simply by what is eye visible. When you want the best, you should have a strategy of finding such a stone which correlates to the way diamonds are graded and only depend on eye visibility when it become eye apparent.

 

[rogue]

Excellent responses, thank you. Dave, invaluable!

 

[Paul-Antwerp]

Date: 8/31/2006 8:59:42 AM
Author: oldminer
''Are there any circumstances that would cause a stone with both 1) good angles and other cut criteria (as determined by the HCA or other accepted measurements) and 2) excellent symmetry to have poor light performance?''


Yes. This is the point that I have been attempting to press forward on in some earlier postings.
You can have what appear to be ''good angles'' to measuring devices that are still painted or dug out in ways that might have a pronounced effect on performance. Some of these changles are nearly imperceptble to human vision and most measuring devices, but quite dramatric at the extremes when it comes to the final look of the diamond.

A stone with excellent symmetry may have problems with light performance due to even small anglular changes or small facet length changes. Symmetry does creat better contrast/intensity, but it is not the only variable necessary for beauty. It is an important component of the situation.

Please consider the more likely occurence of minor cloudiness or UV fluorescence which also could cause a measureable change in light performance, too. Diamonds with large, or dark inclusions may have altered light perfromance in spite of their fine cut details.

In other words, it is a complex subject. Some degradation of light behavior may be measured before it becomes eye visible. This follows the typical way diamonds are graded by very fine increments and not simply by what is eye visible. When you want the best, you should have a strategy of finding such a stone which correlates to the way diamonds are graded and only depend on eye visibility when it become eye apparent.

Dave, I truly wonder whether this is the point that you have been trying to make.

May I ask you what ''excellent'' symmetry you are talking about, if you are refering to small angular changes or facet length changes? I do not think that rogue''s question is refering to the symmetry as it is graded by labs, but to geometrical symmetry, with as close as possible 3-dimensional position and size of all facets.

Your comment on clarity and UV is really going too far, and has absolutely no ''correlation'' to the questions asked.

Are you implying that a SI1 with excellent proportions and symmetry will have less light performance than an IF with ''mediocre'' proportions and symmetry?
Are you implying the same for a stone with Medium fluorescence (super-ideal) compared to a mediocre cut without fluorescence?

Or are you saying that 9.9 is less than 8.5?

It seems that ''your point'' is rather demagogic to me, and living in a city where, sadly enough, the extreme right-wing party gets 30% of the vote, I have to struggle against such demagogy all the time. Like now.

Live long,

 

[oldminer]

Date: 8/31/2006 8:59:42 AM
Author: oldminer
'Are there any circumstances that would cause a stone with both 1) good angles and other cut criteria (as determined by the HCA or other accepted measurements) and 2) excellent symmetry to have poor light performance?'
xxxxxxxxxxxxxxxxxxxxxxxxxx


Dave, I truly wonder whether this is the point that you have been trying to make.

May I ask you what 'excellent' symmetry you are talking about, if you are referring to small angular changes or facet length changes? I do not think that rogue's question is referring to the symmetry as it is graded by labs, but to geometrical symmetry, with as close as possible 3-dimensional position and size of all facets.

I am thinking of the total geometric symmetry issue, not symmetry "lite" as used by the labs propose.

Your comment on clarity and UV is really going too far, and has absolutely no 'correlation' to the questions asked.

"any circumstances that would cause a stone with both"

Note, it says ANY CIRCUMSTANCES. I have given those I feel applicable.

Are you implying that a SI1 with excellent proportions and symmetry will have less light performance than an IF with 'mediocre' proportions and symmetry?

NO, I have never implied that. GIA or AGS may well predict light performance of an SI1 to be better than it actually is. An IF with no strong UV will be correctly predicted within the accuracy range.

Are you implying the same for a stone with Medium fluorescence (super-ideal) compared to a mediocre cut without fluorescence?

A super Ideal will always outperform a mediocre cut unless the UV fluorescence of the super ideal is extreme. I doubt such a situation exists and again, I have never implied this.

Or are you saying that 9.9 is less than 8.5?

Even you and I agree on this. I leave it to physicists to prove otherwise. I will say that a 190 Brilliance on the ImaGem is less than a 200 on the ImaGem. I doubt people can see these ten points difference in average gray scale values, but it may be a true occurrence and pertain to the underlying value of a given stone, visible or not to the unaided eye.

It seems that 'your point' is rather demagogic to me, and living in a city where, sadly enough, the extreme right-wing party gets 30% of the vote, I have to struggle against such demagogy all the time. Like now.

Well, you have only 30% wacko voters where you are. Here the wacko's are in office and we have the entire world second guessing us. I guess we have 51% problem extremists here, so you still need 21% more to get even in unpleasant politics. Many Americans are truly glad that there are term limits in certain political offices. It helps the system to adjust and survive in the long run. I see myself as open to change and learning. You have been instrumental in improving my appreciation of diamond grading by your patient explanation of depth percentage and the new AGS princess cut.

The big concern is to help the trade and consumers to buy a great diamond that suits their needs and budget. Both of us do this every day. I try so very hard not to become an extremist that I suppose in some people's mind's I am one. It is unintentional.

 

[Paul-Antwerp]

Hey Dave,

Sorry for the slip of the tongue.

However, I think that we are talking on totally different levels here, and it creates confusion.

The way I see it, you are talking about differences, measured by a machine, which you doubt are visible to the eye. Causes, according to you, are clarity, fluorescence and/or colour of the stone.

I am talking about differences, caused by extreme geometrical symmetry, which are visible to the untrained eye. There have been pepsi-tests of Infinity-stones, compared to other branded stones, and I myself was surprised how well the consumer could describe the difference in light behaviour between the Infinity and the other stones.

Live long,

 

[strmrdr]

lets go back to the basics there are 3 different symmetry measurements/grades/ratings commonly talked about here relating to diamonds.
They are:

lab symmetry = do the diamond meet points match up? this is graded by eye using 10x magnification.

optical symmetry = are all the facets working together as a team and well matched to one another? This is measured by using a h&a viewer and looking at the hearts image.

physical symmetry = how tight the variations are one like facet to the next. ie: how identical the facets are to other facets of the same type. This is best measured today by using a helium scanner.

physical symmetry is what Paul is talking about in this thread.


.................
opinion here:
The very best diamonds have excellent results in all 3 tests.
I like diamonds with very high physical symmetry it can be argued all day if beyond a certain point it matters but to me its amazing that a complex 3d object was cut with such skill that it has so little variation.
Beyond any and all light performance considerations that means something to me :}