Experts needed on Diamond Proportions.

[Gypsy]

http://www.gia.edu/cs/Satellite?reportno=1162298360&childpagename=GIA%2FPage%2FReportCheck&pagename=GIA%2FDispatcher&c=Page&cid=1355954554547

This is a GIA Very Good Cut diamond that passes the HCA with a .8 FIC. the crown is really high, but it is balanced with the pavillion angle. I have no experience in what this will mean for actual in person performance, except to assume it will have a lot of fire.

No IS is available.
The symmetry is only good on it. And the color is L.
Can you please let me know what the performance of this diamond would be in person?

Is it one of the rare, nice, VG cut diamonds from GIA?

Or are there flags or compromises to the performance.

 

[Rockdiamond]

http://www.gia.edu/cs/Satellite?reportno=1162298360&childpagename=GIA%2FPage%2FReportCheck&pagename=GIA%2FDispatcher&c=Page&cid=1355954554547

This is a GIA Very Good Cut diamond that passes the HCA with a .8 FIC. the crown is really high, but it is balanced with the pavillion angle. I have no experience in what this will mean for actual in person performance, except to assume it will have a lot of fire.

No IS is available.
The symmetry is only good on it. And the color is L.
Can you please let me know what the performance of this diamond would be in person?

Is it one of the rare, nice, VG cut diamonds from GIA?

Or are there flags or compromises to the performance.

It's far more rare to find a VG cut grade diamond with obvious performance problems.
Nice looking VG cut grade stones are not rare.

 

[diamondseeker2006]

Absolutely not an expert but thin to thick girdle and good symmetry would be a concern to me regardless of the HCA score.

 

[Texas Leaguer]

Although the crown/pavilion combo could theoretically reach ideal, the short star/lower will cause LP problems. The fact that it gets dinged for meet point symmetry also suggests likely 3d misaligment. GIA rounding also introduces some unknowns. Thick girdle and medium culet are additional issues. The stone is probably attractive, but not a top performer by any means.

 

[Gypsy]

Although the crown/pavilion combo could theoretically reach ideal, the short star/lower will cause LP problems. The fact that it gets dinged for meet point symmetry also suggests likely 3d misaligment. GIA rounding also introduces some unknowns. Thick girdle and medium culet are additional issues. The stone is probably attractive, but not a top performer by any means.

This is exactly what I needed, thank you TL. Clear and concise.

I know this is probably a hard follow up question to answer but: do the factors above mean that that stone would be discounted as compared with other diamonds with similar specs for weight, cut, and color/clarity?

 

[Texas Leaguer]

Although the crown/pavilion combo could theoretically reach ideal, the short star/lower will cause LP problems. The fact that it gets dinged for meet point symmetry also suggests likely 3d misaligment. GIA rounding also introduces some unknowns. Thick girdle and medium culet are additional issues. The stone is probably attractive, but not a top performer by any means.

This is exactly what I needed, thank you TL. Clear and concise.

I know this is probably a hard follow up question to answer but: do the factors above mean that that stone would be discounted as compared with other diamonds with similar specs for weight, cut, and color/clarity?

Hard to say Gypsy. Not necessarily. For the buyer willing to compromise on cut and color, the proof will be in the putting. That is, if it looks beautiful in real life the merchant can probably get his price for it. Clearly it would be discounted relative to an ideal cut.

 

[Gypsy]

Thanks TL. Very helpful.

 

[carlyt86]

Thank you I have a long day of diamond viewing tomorrow!

 

[carlyt86]

Sorry can I ask what is LP problems?

 

[Gypsy]

Light Performance.

 

[Gypsy]

Your budget can easily get you a top performing stone with higher color.

I highly suggest you keep your wallet firmly closed through any browsing you do, and not be pressured into anything.
Why?
Because there are a lot of stones in your budget and with great specs. Take your time, take a deep breath, and help us make sure you get the best value and nicest stone for your money that you can. That's what we are here for.

 

[Karl_K]

I'm no expert, but I thought I'd share the 57 table cut maps from GIA and AGS.
Your angles fall in "Very Good" for GIA, and "Ideal" for AGS

With a 70% lower the general combo would not get ideal LP at ags and depending on the range and rounding the pavilion angle may also disqualify it for an ideal LP grade. It would be kicked out of ags0 overall grade by the finish grades at any rate. So overall there is zero chance of it getting ags0.
With a gia 80% lower there would be a chance of a fairly nice diamond within the gia rounding but also a chance of it not being so nice.

There is also a chance it would not get vvs2 clarity at AGS or if resubmitted to GIA.
There is a lot of stuff going on a for a vvs2 grade.

 

[Gypsy]

There is also a chance it would not get vvs2 clarity at AGS or if resubmitted to GIA.
There is a lot of stuff going on a for a vvs2 grade.

That's what I thought too. I saw the list of inclusions and did a total double take at the VVS rating. I've never seen a list of inclusions that long on a VVS stone.

 

[Rockdiamond]

In my experience on the market, most stones with the "VG" cut grade trade at a lower prices than a Triple EX especially true on any sale having anything to do with the Internet- or to any knowledgeable buyer.
None of us have seen the stone – but it's quite possible that the problems being referred to here would be quite minor,from a visual standpoint- and the price discount could be substantial.
When I'm looking for a good value in a round diamond, I do not overlook VG cut grade stones. In many cases you would need a loupe or microscope to see the difference between a VG and an EX cut grade – of course not all cases but many. There are also many cases of different lower girdle/ table/ depth combinations that would make a VG cut grade stone more attractive to a given buyer then another EX cut grade.

 

[Rockdiamond]

Sorry can I ask what is LP problems?

This is really a great question… Bryan could you please specify what sort of "lp problems" you're talking about?

Carly-"light performance" is an advertising term – there is no scientific calibration behind it.

 

[John P]

Is it one of the rare, nice, VG cut diamonds from GIA?

Yes. With some caveats.

This topheavy make is the opposite of deep pavilion, shallow crown. I see these makes with some regularity, usually graded by EGL.

RE the performance question: If it had longer lower-halves, and was cut with nice 3D precision, it could be very appealing and might just squeak into AGS0 in light performance. The problem with shallow pavilion mains is the chance of increased head-shadow (obstruction). A 40.4 goes dark if the crown is shallow. But here the CA+table here make a heavier, complimentary top. Unfortunately the short lower half choice widens the pavilion mains, which increases the obstruction problem. Odd choice.



There is no way to know how this specific diamond would score at AGSL. As the simulation above shows, those base numbers can work (AGS 0 light performance). But I would never bet on that happening, because the sim is based on the grading report's limited data and presumes a perfect wire-frame which is totally improbable IRL. Thus it's a "best-case" scenario and, as Bryan and Karl noted, the 2D symmetry grade is not a good indicator of the 3D cut-precision needed to sustain top values in their scientific light performance metric. Yes, I said scientific.

http://www.diamonds.net/news/NewsItem.aspx?ArticleID=19529

 

[Rockdiamond]

Hi John
The way I see it the problem is one of terminology.
I am a believer in science.
AGSL uses scientific methodology in assigning the cut grades.
However so does GIA – and they have reached different conclusions.
Part of the reason is that the darkness you're referring to, or the obstruction problem you're referring to, might not be a visual problemsl to many observers.
My point is that there are two grades- GIA and AGSL, both scientifically obtained reaching different conclusions. What that means is that the cut grades themselves are subjective as opposed to scientific measurements.
The term light performance therefore is just ones opinion of how the diamond reacts to light.
Therefore, claims of "superior light performance" are merely advertising slogans.
There's nothing wrong with advertising slogans – tying it with science in an objective discussion like this one is misleading.
The whole point of a form like this is the cut through advertising slogans and give consumers more balanced look.

 

[BrownyJones]

Hi John Pollard, this is a fascinating discussion since I am a physicist and appreciate attempts at quantifying these qualifications. When you have some time, could you use your light performance tool and calculate the theoretical light performance of these 2 stones I am looking at? Thanks John.

the specs are on the link below.
[URL='https://www.pricescope.com/community/threads/and-its-down-to-these-2-advice-needed-please.210468/']https://www.pricescope.com/community/threads/and-its-down-to-these-2-advice-needed-please.210468/[/URL]

 

[Texas Leaguer]

Hi John
The way I see it the problem is one of terminology.
I am a believer in science.
AGSL uses scientific methodology in assigning the cut grades.
However so does GIA – and they have reached different conclusions.
Part of the reason is that the darkness you're referring to, or the obstruction problem you're referring to, might not be a visual problemsl to many observers.
My point is that there are two grades- GIA and AGSL, both scientifically obtained reaching different conclusions. What that means is that the cut grades themselves are subjective as opposed to scientific measurements.
The term light performance therefore is just ones opinion of how the diamond reacts to light.
Therefore, claims of "superior light performance" are merely advertising slogans.
There's nothing wrong with advertising slogans – tying it with science in an objective discussion like this one is misleading.
The whole point of a form like this is the cut through advertising slogans and give consumers more balanced look.

RD, forgive me for being skeptical that you are a believer in science. If you were, you would have made an effort to understand the science behind AGSL light performance based cut quality grading long ago. The link at the end of the article that John referenced is not working but you can find it here.
http://www.agslab.com/spie/spie_lo_res.pdf

Instead of criticizing any particular aspect of the AGSL science, you regularly disparage the whole concept that light rays which obey the laws of physics can be quantified and understood in terms of their various impacts on the eye of the observer. Many people here have brought forth sound scientific arguments to demonstrate that the AGSL system is not perfect. You are not one of them. All you have done (for years) is repeat the same unproductive diatribe about light performance being a "marketing slogan" and grading systems being "subjective".

AGSL will be the first to tell you their system is not perfect. It will most likely be improved in the future by AGSL and/or other entities. But it is based upon vetted science. And that will not change no matter how many times you throw your wet blanket over thread after thread.

 

[John P]

Hi John Pollard, this is a fascinating discussion since I am a physicist and appreciate attempts at quantifying these qualifications. When you have some time, could you use your light performance tool and calculate the theoretical light performance of these 2 stones I am looking at? Thanks John.

the specs are on the link below.
[URL='https://www.pricescope.com/community/threads/and-its-down-to-these-2-advice-needed-please.210468/']https://www.pricescope.com/community/threads/and-its-down-to-these-2-advice-needed-please.210468/[/URL]

Browny, punching those limited numbers into software is much like an HCA result. It's somewhat predictive but there are not enough data points to decide between them. I'll elaborate over in that thread.

 

[denverappraiser]

Indeed, GIA and AGS have approached the concept of performance very differently.

The AGS plan is to make a computer model of the stone and then raytrace theoretical light paths from a variety of different sources. The results are then divided into 10 groups and ranked accordingly.

The GIA approach was to have real humans look at stones that the scientists knew a lot about and force rank them in terms of beauty. The observers weren't told any of the facts. They were then grouped into 5 categories based on ones that a lot of people liked vs. ones that hardly anyone liked. A subject stone is computer modeled and given the same grade as the most similar stone in the test group.

Yes, both are science. And both have logic behind them. GIA sees that what is being evaluated is 'beauty', a notoriously squishy concept, and chose a way to attach a relatively unbiased score to that. AGSL approached it as a physics question. They established a set of parameters that they deemed reasonable (like that the optimum viewing angle is from directly above) and proceeded accordingly.

They're not incompatible, but no they're not the same. I tend to like the AGS approach but I do get why GIA does it the way they do. Diamonds are art and beauty is in the eye of the beholder. One of the lessons from the GIA study was that SOME people liked best stones that GIA ended up calling 'good'. Does that make those people wrong?

 

[John P]

The AGS plan is to make a computer model of the stone and then raytrace theoretical light paths from a variety of different sources. The results are then divided into 10 groups and ranked accordingly.

The GIA approach was to have real humans look at stones that the scientists knew a lot about and force rank them in terms of beauty. The observers weren't told any of the facts. They were then grouped into 5 categories based on ones that a lot of people liked vs. ones that hardly anyone liked. A subject stone is computer modeled and given the same grade as the most similar stone in the test group.

Neil, I want to address the comment highlighted in red.

A full computer model is not in play. The averaged and rounded 2D measurements dictate the proportions-grade. Any diamond with a pavilion angle <40.6 will be VG at best, period...there is no recourse...even if cutting choices involving crown angles, brillianteering and 3D optical-precision improve the diamond, and make it more appealing than many GIA EX choices. That's blindly penal.

On the other side, any diamond sent to GIA with averages of 59-41-36-70-50 is automatically "EX," even though precise details of minor-facets, brillianteering and 3D optical-precision can make one such example attractive and another go quite dark at the edges in low light. Two different-looking diamonds. Same grade in every case because the details are distilled to a few data points.

Bluntly put: GIA uses 2D averages and looks no further. But AGSL's metric is diamond-specific; two diamonds that have the same 2D averages may score differently because the lab takes all 57 facets into consideration. This creates a huge gap in accuracy and consistency on a diamond by diamond basis.

 

[Rockdiamond]

Excellent post Neil- your description of the different approaches was spot on.

Bryan - I have taken a lot of time learning about AGSL grading.
In my opinion the weak point in any system using ray tracing is the inability to compensate for real life viewing.
To calibrate the Ray tracing system certain fixed lighting parameters are necessary.
Light does not behave that way in real life.
For example - "our world is lit from above" is an irrelevant assumption even if it's true.
Diamonds are so rarely viewed with the table parallel to the floor.
Another example- The benefits of balanced leakage in a diamond are totally underrated in a ray tracing system. Once light starts bouncing around in the stone the variation becomes exponentially more complex to analyze using Ray tracing.
Since I am usually the sole voice defending other methods of assessment that does not mean I don't respect AGSL cut grading.
Without a doubt AGS000 is the most consistent cut grade in terms of the appearance of the stones.
That's great if one wants that look.
It takes expertise and costs money to produce that level of perfection in cutting. I appreciate the value of such precision.
From my perspective cut grading can be compared to color grading.
Is a D " better" than a J?
As we all know some people are so color sensitive that they find a G to show too much yellow- and other people who actually prefer the soft white of J over a colorless diamond.
The notion that it's rare to find an attractive GIA VG cut graded stone does not hold true in real life in my experience. I do get to see quite a few.
I adore cut, respect cut, recognize excellent cut.
But it would benefit buyers to look at a variety of diamonds with varying cut grade - presented in an impartial manner- to see how much preference to place on cut.

 

[denverappraiser]

John,

AGS has better models. I agree, but GIA's approach is still based on modeling. Neither one is making the determination by looking at diamonds, which is how they approach clarity, color, polish, fluorescence and so on, they get it by computers looking at models that are constructed from scans.

 

[Texas Leaguer]

John,

AGS has better models. I agree, but GIA's approach is still based on modeling. Neither one is making the determination by looking at diamonds, which is how they approach clarity, color, polish, fluorescence and so on, they get it by computers looking at models that are constructed from scans.

Agreed that both AGS and GIA cut grading systems are based on modeling and sophisticated ray tracing. GIA used it upfront as a research tool in order to create their tables which are then matched to the averaged and rounded data from the scan of the diamond being graded. But as John points out AGS uses the actual scan of the individual diamond to ray trace and grade the very diamond being reported on. For this reason alone the AGS system is far more accurate in terms of analyzing the specific diamond on the report. The ray trace takes into account every facet, its dimensions, angles and azimuths.

The science to do this was available to both GIA and AGS. AGS, relatively new and already known for cut quality analysis and catering to a clientele extremely interested in this aspect of diamond value, was unimpeded in their pursuit of the most critical analysis possible. GIA on the other hand, had a huge existing constituency of manufacturers, dealers and jewelers with huge existing inventories of diamonds cut primarily for weight. It was therefore not politically feasible for GIA to roll out a system that was too strict. They therefore decided upon broad categories of cut quality and a table-based assessment using their rounding and averaging. They validated their construct with a very large number of human observations. In general it has been a very successful system, helpful to consumers and prompting significant improvements in the cut quality of rounds on the market today.

However, the GIA system is under-serving a growing portion of the consumer market hungry for the most accurate information available on the cut quality of the diamonds they are shopping. And that is where AGSL finds a receptive audience.

 

[Rockdiamond]

John,

AGS has better models. I agree, but GIA's approach is still based on modeling. Neither one is making the determination by looking at diamonds, which is how they approach clarity, color, polish, fluorescence and so on, they get it by computers looking at models that are constructed from scans.

Agreed that both AGS and GIA cut grading systems are based on modeling and sophisticated ray tracing. GIA used it upfront as a research tool in order to create their tables which are then matched to the averaged and rounded data from the scan of the diamond being graded. But as John points out AGS uses the actual scan of the individual diamond to ray trace and grade the very diamond being reported on. For this reason alone the AGS system is far more accurate in terms of analyzing the specific diamond on the report. The ray trace takes into account every facet, its dimensions, angles and azimuths.

The science to do this was available to both GIA and AGS. AGS, relatively new and already known for cut quality analysis and catering to a clientele extremely interested in this aspect of diamond value, was unimpeded in their pursuit of the most critical analysis possible. GIA on the other hand, had a huge existing constituency of manufacturers, dealers and jewelers with huge existing inventories of diamonds cut primarily for weight. It was therefore not politically feasible for GIA to roll out a system that was too strict. They therefore decided upon broad categories of cut quality and a table-based assessment using their rounding and averaging. They validated their construct with a very large number of human observations. In general it has been a very successful system, helpful to consumers and prompting significant improvements in the cut quality of rounds on the market today.

However, the GIA system is under-serving a growing portion of the consumer market hungry for the most accurate information available on the cut quality of the diamonds they are shopping. And that is where AGSL finds a receptive audience.

Bryan- you're quick to toss insults but refuse to respond to a reasonable questioning of how pragmatic Ray tracing is - particularly if Ray tracing is used without human observation.
Now you're claiming to understand the motives behind GIA cut grading. Were you included in their deliberations? Did you personally inspect dealers inventories to ascertain what percentage of stones were cut placing too much priority on weight retention?
Maybe the reasoning behind human observation was that such testing is crucial to finding out what is actually desirable to human eyes.


I am also genuinely curious to the observations of those who actually look at diamonds for a living to discuss how many badly cut VG cut grade stones they see.

 

[denverappraiser]

People are eager to compare the two labs and routinely ask which one is better.

This is a false dilemma. Although there's a fair amount of overlap in the top grades, they aren't answering the same questions.

I'm more of a physics sort of guy and I definitely like the AGS strategy but GIA does have a point. One of the assumptions in the AGS model is that the viewing environment is with one eye from directly over head with the light source also coming from directly overhead. This is not a 'real world' condition. GIA doesn't get a free pass either. Their study was fundamentally measuring popularity, not performance. These are not synonyms. Popularity is of enormous interest to dealers who want to invest their money in things that are likely to sell quickly for them, but it's far more problematic for consumers trying to decide which one is 'best'.

 

[teobdl]

At what distance would one start to have an "obstruction problem"? If its not a problem at more than 25 cm, I wouldn't be concerned. Even then, the instances in which this is a real scenario are scarce.

Assuming obstruction isn't an issue (and that the person knows this isn't a super ideal), I don't see a compelling argument here to rule this diamond out on cut alone.

Gypsy asked what the "performance" of this diamond would be like. I think it's reasonable to say that it would be a beautiful, fiery diamond that looks incredible in spot lights. Probably doesn't look so good in diffuse lighting, but that may also be my bias against middle alphabet diamonds swallow up diffuse light when you look at them at any angle except perpendicular to the table.

 

[John P]

At what distance would one start to have an "obstruction problem"? If its not a problem at more than 25 cm, I wouldn't be concerned. Even then, the instances in which this is a real scenario are scarce.

Assuming obstruction isn't an issue (and that the person knows this isn't a super ideal), I don't see a compelling argument here to rule this diamond out on cut alone.

It's at the heart of the matter: GIA's uses a notably larger obstruction premise in their metric than AGSL. That allows a number of steep-deep proportion sets into the EX grade which get too dark under the table and at the edges in low-light environments for many peoples' tastes. It also automatically knocks diamonds with PA under 40.6 out of EX, even those able to return "Ideal" performance values in AGSL's more specific metric.

 

[Rockdiamond]

Neil- my take on the popularity question would pose it differently.
My problem with calling an AGSL cut grade "best" is exactly your concern about the usefulness to consumers.
As you point out both systems have their advantages- I advocate a more objective comparison of both in these discussions.
For exampe- take the rounding issue- which is clear cut advantage to AGSL for a physics guy or gal.
But how much of an issue for someone wanting a good looking well cut diamond?
Put another way- say your cars speedometer was programmed for inches per hour versus miles per hour. The more precise measurement is clearly more accurate -but to what end?