Who truly understands, and can explain, optical symmetry?

[tenbinko]

Vendors and experts, including John, have criticized or at least pointed out the fact that when labs grade symmetry, it's 2-D symmetry that they grade and "Excellent" symmetry doesn't necessarily mean precise cutting from a 3-D perspective, and I've also read at various places about the superiority of 3-D precision cutting (including at http://goodoldgold.com/4Cs/NewCutGrading/OpticalSymmetry).

I understand that the super ideals must have perfect HA patterns and the lower girdles, for instance, must be cut to 77%, and that's what I've seen on reports for BGD Signature or WF's ACA diamonds, or if it's not 77%, it's 76%, which is close enough. But what would be the problem with 80% across the board? We might not like the resulting HA patterns, but still, that would yield perfect optical symmetry, would it not?

If you look at the GIA symmetry grading document I attached, it seems that it's fairly comprehensive... and some pictures do suggest 3-D symmetry grading, as opposed to 2-D grading... and even if it's all 2-D, if you've got enough views, you've got the 3-D picture, pretty much.

After having bought diamonds, it still bothers me that I can't explain to others why certain diamonds would have nice optical symmetry and others might not... I guess I just don't understand why... it seems to me that what GIA does is sufficient, at least from the document I've attached.

 

[Lula]

Re: Who truly understands, and can explain, optical symmetry

I find it a pretty confusing topic. Here's a thread on the topic from a few years ago that I started: https://www.pricescope.com/communit...d-physical-symmetry-and-effect-on-cut.125328/

John Pollard, Rhino (Jonathan) and Gary and Serg, along with some of the other vendors and appraisers who post on PS may be able to answer your question.

Oh, geez, forgot to mention Karl -- he could explain it. Hopefully he'll see this thread and respond.

 

[yssie]

Re: Who truly understands, and can explain, optical symmetry

Some vendors require that branded stones be cut to fit into a tight range of proportions - that results in stones w/ very similar character that all have the same sort of hearts and arrows patterns and quality of light return. Other vendors require only that patterning be radially symmetric, not that proportions conform to certain constraints, so those vendors' H&A stones may show a variety of patterns and type of light return.

Optical symmetry *is* a function of the phsycial "facet meet" symmetry that the labs grade - in part, but not completely. The pattern of facets you see when you look into the stone is a pattern of virtual facets that yield primary/secondary/tertiary refraction patterns, and each of those tiny little facets behaves like a physical facet (returns light, obstructs, or leaks)... obviously asymmetries in the physical faceting that creates the architecture that's reflected internally again and again will impact the symmetry of the virtual facet pattern, but there can be cases (they're rare) when symmetry is downgraded on the report for reasons that have minimal impact on VF symmetry. I don't think I've seen more than a couple, maybe - the one that I remember best was an Infinity w/ VG sym and picture perfect hearts and arrows.

Of course part of that is the fact that you're dealing wth specialty brands that have to meet other obligations - brands like ACA and BGD signatures are sold with AGSL DQDs and as part of brand promotion & continuity require that stones earn the ideal 0 grade, so you won't ever get the opportunity to see H&As w/ non-ideal cut grades from them... that sort of thing! HRD issues (issued?) a H&A report w/ guidelines for grading H&A (by their definition, which may be different from others' definitions...) http://www.hrd.be/media/24344/hearts_arrows_guidelines.pdf

Ditto Lula, one of the tradepeople can give you a more thorough explanation. In the meantime check out the thread she linked, and this article for a good intro: http://www.agslab.com/spie/spie_lo_res.pdf

 

[diamonds-are-friends]

Re: Who truly understands, and can explain, optical symmetry

grades grade symmetry with a tolerance and how facet junctions meet. Optical symmetry requires a precise angle to how each of the facets are proportioned.

Let me explain it in laymans terms.

If the score for an exam is 100 marks and the grade for an A is 80 marks and above. A student (diamond) with 81 marks is graded A (Excellent)

Another student is so precise in his answers and scores a 97 marks. The school (GIA) assigns the same grade of A.

For perfect optical symmetry to take place, this tolerance must be very small. The first diamond most properly had less consistent optical symmetry than the second stone.

 

[Lula]

Re: Who truly understands, and can explain, optical symmetry

Here's another old thread with a great comparison photo in it: https://www.pricescope.com/community/threads/is-optical-symmetry-needless-perfection.10184/

 

[Karl_K]

Re: Who truly understands, and can explain, optical symmetry

There are 2 different but related things being discussed here:
optical symmetry and h&a.
h&a images is one way of showing optical symmetry with arbitrary standards.
Take 3 diamonds with the same level of optical symmetry, one 77% lowers, one 60% lowers, and one 85% lowers.
All 3 have the same level of optical symmetry but only the 77% lowers would form the perfect hearts of h&a.
Typically RBs with lowers from 74%-75% to 80%-81% can be considered h&a when optical symmetry is high depending on who you ask.
That does not mean diamonds outside that range(or even other shapes) can not have high optical symmetry.

That help?

 

[Karl_K]

Re: Who truly understands, and can explain, optical symmetry

After having bought diamonds, it still bothers me that I can't explain to others why certain diamonds would have nice optical symmetry and others might not... I guess I just don't understand why...

The simple answer is that a diamond is a 3d object and how well the facets(reflectors) align together in 3d determines optical symmetry.

 

[WinkHPD]

Re: Who truly understands, and can explain, optical symmetry

Obtaining perfect optical symmetry can also be made more difficult if there are optical deviations within the diamond. It takes a master cutter who makes constant tiny adjustments to the diamond as it is being faceted to bring it all together. With some crystals it is simply not possible and the diamond is sold as a lesser cut diamond, never to wear the brand of the house that was trying to make it special.

Of course the public never hears of these diamonds until they are sold as the unremarkable diamonds that they are, which are still pretty remarkable for being diamonds.

Wink

 

[WinkHPD]

Re: Who truly understands, and can explain, optical symmetry

One of the reasons for wanting optical symmetry is that this will result in the largest virtual facets for the diamond of its size and thus more visual scintillation events.

If you were to look at the wire frame of virtual facets posted above and observe how it changed through 40 degrees if tilt from side to side and if you had a LONG TIME to count and could see minute facets you would see that there are close to 200,000 scintillation events as the stone rotates.

Of these, in say a 1 carat stone that is cut to Hearts and Arrows standards eight of those would be Large scintillation events, there will be many more Medium events, many times more Small to Very Small events, and while there are perhaps somewhere in the neighborhood of eight to ten thousand visual optical events the vast majority are not perceivable to the unaided human eye. In a half carat stone there might actually be no large events and in a two carat stone there may be eight Very Large events. That is one of the reasons that the eye loves larger stones, they just have more discernible scintillation events.

With precisely cut stones that have top of the line optical symmetry the resultant events are crisper and larger and just look better than the smaller slightly fuzzier events of stones that might actually have the coveted AGS 0 cut grade, but just barely.

So, I would respectfully disagree with the comment that what GIA does is good enough. It is not, and in fact their papers reward diamonds that are "good enough" by calling them excellent, making it impossible for those who strive for the best to make a good decision even of which stones to take a look at just by the paper alone.

There are many many companies figuring out how to cut Hearts and Arrows "good enough" to get the GIA Excellent and AGS triple 0 cut grade by the skin of their teeth, there are only a few that cut to the top of the AGS triple 0 cut grade envelope. For some people, only those top of the grade diamonds are "good enough."

Wink

 

[tenbinko]

Re: Who truly understands, and can explain, optical symmetry

Thanks, everyone.

I was unsure about this whole symmetry thing because of what GIA said below:
_____________________________________________________________________________________________________________
"Symmetry features can be subdivided into two types: proportion-related and facet-related. Due to a polished diamond’s three-dimensional nature, the presence of one symmetry feature may be linked to others. GIA Laboratory graders consider the extent and visual appearance of any symmetry features present."

AND

"Excellent: ranges from no symmetry features to minute symmetry features that can be viewed face-up with difficulty at 10X magnification. Some typical features that would be allowed in the Excellent category include misaligned, misshapen, non-pointed, or extra facets that are barely visible."
_____________________________________________________________________________________________________________

That's why I thought GIA would grade from a 3-D perspective, as opposed to 2-D, which everybody else claimed, and because an Excellent symmetry grade would mean "minute" features that would be "barely visible" under 10x magnification, I never imagined it would lead to such a drastic difference of, say, 81% vs. 97%, in an analogy provided above. I thought, and still feel, that it's more likely the difference between 97% and 100%, at least according to the wording in GIA's cut grade explanation documents.

But I've learned quite a bit from your answers and the links... for larger diamonds, symmetry might not be the best thing; asymmetry would add to scintillation and decrease fire, but if somebody's after scintillation above all else, then perfect symmetry might not be the answer... And inclusions would affect optical symmetry, of course...

I still wish I could find out more... like what would make a diamond receive an AGS-0 and/or GIA-Ex for symmetry and still have terrible optical symmetry, provided that there are no inclusions?

Somebody mentioned very briefly in a response in one thread about slopes and azimuth angles... is there a more detailed explanation of that somewhere? But it seems that would be included in GIA's symmetry grading, though... say you've got a star length of 45% somewhere though it's 50% everywhere else, it'd affect the slopes/angles, but not so much that you'd see no HA pattern or excellent optical symmetry whatsoever, right?

 

[WinkHPD]

Re: Who truly understands, and can explain, optical symmetry

I was going to refer people to https://www.pricescope.com/community/threads/interview-with-peter-yantzer-consumer-issues.117241/ for the third video of Pete Yantzer talking about scintillation events, but I notice that the videos are no longer working.

Bummer. Andre, if you want them, I still have copies.

Wink

 

[Karl_K]

Re: Who truly understands, and can explain, optical symmetry

I have seen diamonds with GIA vg/vg polish/symmetry with picture perfect h&a images and Garry H.(hca inventor) has stated in the past he has seen good/good with picture perfect h&a images.

 

[Paul-Antwerp]

Re: Who truly understands, and can explain, optical symmetry

The 'art' of cutting is basically in positioning the various facets in such a way that they optimally interact. With regards to symmetry in this exercise, there are various levels of explaining, judging and/or grading symmetry, which is extremely confusing.

1. True symmetry in positioning the facets is a factor of how well the cutters control their tools, and how dedicated they are to spend more time on getting that true symmetry. As a result, with all facets perfectly positioned, one gets a diamond without or with less 'extra' virtual facets of very small size. These 'extra' virtual facets are a negative, since they produce flashes of lower quality, not noticeable, and because they are unintended, their operation is also not directed. Furthermore, they reduce the intensity of the flash as the intended bigger virtual facet goes from 'off' to 'on' (the delta, if you wish).

2. Observed optical symmetry as in H&A-viewers is illuminating but has its limits. The viewer to start is a very simple device, separating the perimeter of a diamond basically in two colors only, and as a result, the resulting information is also a simplification in the sense that there must be lost information. There not being a standard in the design of H&A-viewers does not help either.

3. Graded optical symmetry as in H&A-grades of various labs is another story. Generally, these labs have rules to which the observed H&A-pattern should comply, meaning that if that observation passes a minimum-level, the grade is assigned.

4. Graded symmetry as in the symmetry-grade of most major labs is again something completely different. Basically here, facet junctions and other symmetry deviations are judged, with little to no attention on how the facets are positioned versus the other facets.

This difference is confusing in two ways. In a sophisticated discussion on symmetry, I might be talking about level 1 mentioned above, while others treat it as level 2, 3 or 4, leading only to misunderstanding, confusion and eventually total disagreement.

Commercially, the confusion is also high. Depending on a cutter's market, he might well be content with working at the minimum-level of level 4, 3 or 2. Definitely, worldwide, there are various markets, clearly preferring one of these levels. Level 1, with its inherent difficulty of measuring afterwards is a bit of an unknown zone, and thus not a target for most cutters.

In summary, as you can see in this thread, your question to understand and explain optical symmetry leads to a very confusing exchange, eventually ending in a division between believers and non-believers. I hope that my view, as a believer, nevertheless was somewhat illuminating.

Live long,