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Usefulness of diamond parametric screening tools

oldminer

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There was a recent topic, which I felt compelled to enter, with some numerical and parametric advice in terms of assisting a consumer in selecting what is a top-quality fancy shape diamond. Of course, I was countered by those who readily advise that you can’t choose a diamond by numbers. This is not totally false, but it is only partially true. Diamonds of the highest quality are distinctly “the best” when their light behavior/light return is near maximized but only when COMBINED with parameters indicating the best craftsmanship and make possible.

Light behavior can be viewed and subjectively quantified by I-S or ASET scopes. It can be factually measured, to varying degree of correctness, by digital tools produced by Sarin, Ogi, Imagem, DiamCalc Gemex. It is reliably predicted and graded by the AGS cut system and to a slightly lesser, or more liberal, extent by the GIA cut system.

Parameters and numbers used to screen diamonds for elements of their craftsmanship may not always be used for the above grading of light behavior. These facts may be considered in some systems, but used more for light predictive purposes than for arriving at a grade for the craftsmanship quality of a cut. If you buy a diamond solely based on light behavior grading, you may someday find that while it is a very pretty diamond that does have excellent light return, it may have some problem with elements of its design and fashioning that were never mentioned as being less than “the best”. Over thin, overly thick girdles, too large or too small table, overly deep or overly shallow total depth, or an unusually short or long length to width ratio, overly bulging or shallow curvatures, or atypical asymmetry may exist that does not adversely affect light behavior but still would encourage most every dealer to criticize the diamond’s cut if you wished to sell the stone back into the trade someday. Such defects in cutting can alter the value of a diamond and cause one to seemingly be a better value than another of equal grading when the diamonds are not, in fact, equal in total quality.

Garry’s Holloway’s HCA gives consumers a numerical grade predictive of light return for just round diamonds. It is based on the input of a few parameters of cut. The AGA Cut Class tool give a craftsmanship grade and allows consumers to know the weakest and best elements of many shapes of diamonds. It is not predictive of appearance, other than for round diamonds, but eliminates virtually all shape diamonds with cut problems that might otherwise be overlooked. Diamonds that score very well with AGA Cut Class that also have great light behavior grades are going to be those everyone, dealers and consumers, should say are “the best”.

Part of the reason consumers seek out “the best” cut is to make it difficult to get burnt in a purchase. The dealer nearly always knows more than the consumer. Round and princess cut diamonds have become far less blind a purchase, but the purchase of most fancy shapes is still not a transparent process. With the exception of most Pricescope vendors, most diamond dealers prefer to keep as much of the facts under wraps to the greatest extent possible. We can sympathize and understand their plight. However, transparency is the path we are on here, and have been on, for quite a long time now.

My advice is to select diamonds which are graded nearly the same and adjust for their weight, color, and clarity differences. Then, examine their images, such as ASET and I-S. Then run their numbers with AGA or HCA and look at any nuance differences which show up on the parametric level. These might or might not make a visual difference, but they might be major part of the cause that the asking prices are not equivalent. That is what screening is valuable for. You can choose what you wish, but you ought to understand what you are buying today rather than find out years from now.

An important reminder: Parametric suggestions for diamond screening do not apply to natural fancy color diamonds in the way logic might dictate. Natural fancy colors, even the most common varieties, are relatively rare and their face-up appearance trumps many other considerations of cut craftsmanship and light behavior measurement. Their unique or odd shaping may contribute to substantially more visible color and increase their value. The rarer or more intense the color, the less parametric numbers seem to correspond to pricing. Color rules in these situations.
Near colorless diamonds are the main order of business for most of Pricescope’s consumers and they are the folks being addressed by this topic.
 

Blingalingaling

Brilliant_Rock
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I read the other topic with great interest and I thank you for your additional input with regards to using numeric aids to screen out the "best" stones available to a consumer.
My question is regarding pear shaped stones. I have observed on GIA certs that there are a variety of facet patterns for pear shapes and I'm wondering what the differences are between a stone that is 8-main versus one that has a 5-main cut pavilion or other facet pattern. Both the 8-main and the 5-main, for example, are listed as "pear brilliants" in the cut category of a cert. Does the fact that a particular stone is an 8-main mean that it will potentially reflect light better than a 5-main stone or be more brilliant? Is there any correlation between the type of faceting on the pavilion and the overall beauty of the stone?

Thanks!
 

Karl_K

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I disagree, it works reasonably well with round diamonds that fit the assumption for lowers and griddle because the facets are locked into place by the design.
Not so with fancy cuts.
For example 2 step cut diamonds could have the exact measurements but the facet placement is different this
can result in 2 widely different looking diamonds. It is enough to make a quality difference.
Step cuts can also vary with the number of facets.
Not to mention the l/w ratio and corner size both change the optimal placement and angles of a step cut.
On top of that a step cut could be top of the line, but the consumer may not like that specific look.
There is a huge variation in looks possible in top of the line step cuts. Far larger than with a RB.

Other fancies have many different possible facet configurations all of which have different angle and
placement requirements for top light return.

I could go on all day but my bottom line,,,,
There is no set of numbers that will tell you if a fancy cut diamond has great light return potential.
There are just too many variables.
 

oldminer

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I did not say AGA cut class predicts light return. It helps consumers define properly crafted Diamonds to eliminate those with craftsmanship problems that often get overlooked or dismissed by sellers. You mentioned some issues which also are mostly appearance related and occasionally might reflect on craftsmanship. In diamonds everything's counts. Neither of us want consumers to think it is as simple as it appears on first glance. I have no facts today related on the variety of facet patterns. No doubt, facet count and patterns do change light return. I have no data on how much any pattern is better or worse than another. Maybe someone is studying it but it is loaded with variables.
 

Karl_K

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Oldminer|1478607684|4095110 said:
I did not say AGA cut class predicts light return. It helps consumers define properly crafted Diamonds to eliminate those with craftsmanship problems that often get overlooked or dismissed by sellers. You mentioned some issues which also are mostly appearance related and occasionally might reflect on craftsmanship. In diamonds everything's counts. Neither of us want consumers to think it is as simple as it appears on first glance. I have no facts today related on the variety of facet patterns. No doubt, facet count and patterns do change light return. I have no data on how much any pattern is better or worse than another. Maybe someone is studying it but it is loaded with variables.
Thinking like that is very badly holding back diamond design.
That type of thinking is a major problem for some of the best princess cuts ever cut
and one of the best asscher designs ever cut are penalized by some because the numbers are considered weird even though the craftsmanship, light return and appearance is world class.

Patterns and light return are what the owner experiences without them its just another rock.
They beat numbers on a chart any day.
How are they achieved on a consistent bases? Craftsmanship.
 

Karl_K

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On a more personal note I generally avoid discussing the AGA system
because David is one of the people I respect most in the diamond world.
While it was a good try to quantify the numbers I feel the world has moved on.
 

Paul-Antwerp

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Karl_K|1478611906|4095140 said:
Oldminer|1478607684|4095110 said:
I did not say AGA cut class predicts light return. It helps consumers define properly crafted Diamonds to eliminate those with craftsmanship problems that often get overlooked or dismissed by sellers. You mentioned some issues which also are mostly appearance related and occasionally might reflect on craftsmanship. In diamonds everything's counts. Neither of us want consumers to think it is as simple as it appears on first glance. I have no facts today related on the variety of facet patterns. No doubt, facet count and patterns do change light return. I have no data on how much any pattern is better or worse than another. Maybe someone is studying it but it is loaded with variables.
Thinking like that is very badly holding back diamond design.
That type of thinking is a major problem for some of the best princess cuts ever cut
and one of the best asscher designs ever cut are penalized by some because the numbers are considered weird even though the craftsmanship, light return and appearance is world class.

Patterns and light return are what the owner experiences without them its just another rock.
They beat numbers on a chart any day.
How are they achieved on a consistent bases? Craftsmanship.
I fully agree with you, Karl.

Live long,
 

oldminer

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Asscher cuts are a special cut for a very small number of consumers who simply want that unusual visual look. They like a diamond which has fantastic looks, but is very small visually and is very deeply cut. I suppose you might defend such a stone as perfectly cut, but I think such diamonds have a built in fault of cutting, "looking smaller than most others of similar weight". The consumer must decide, not me.

The vast majority of people want a diamond with a reasonable spread for weight. Over depth decreases visual size. Very thin girdles increase the chance of shipping. Few folks who know this willing opt to take the chance. DIamonds with bulges are not generally as attractive or desirable as those with pretty curvature. People choose based on "looks" in many cases, but need tools to increase their awareness of nuance issues.

I know you disagree with me for whatever reasons you have. No doubt you have lots of knowledge, so it isn't ignorance. You cut the best of the best, so it isn't your diamonds we are even discussing. No one needs to be concerned with the level of your cut stones. You seem to be protecting consumers very well with your production, but so many other sources don't care to your level or make their proiduct nearly as well cut. Those are the ones being addressed.

The very finest diamonds have high light return and have no other cutting faults. Anyone who would come forward and say the only consideration in "cut quality" is light behavior and light return" would be dead wrong. There are less discussed attributes which are important facts and variables. These variables often alter the value, limit the look, create durability issues or impact the visual appearance in some way beyond simple light return. You might say we are arguing over something too minor to discuss, but I think it is something which consumers want to understand and then make their own informed judgment about.

Anyway, I am very happy for this forum to present my point of view and always welcome factual presentations which offer an enlightened and accurate counterpoint. I am sure that all AGS and GIA grading has numerical limitations of many aspects of cut for their best cut grades. That's how Sarin and Ogi are able to calculate cut grades. Those numbers are facts, not idle dreams. Once you know how to get high lght return most any shape can be constucted, by some complex set of numbers, to cre-recreate the diamond over and over.
 

pyramid

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I agree with Oldminer. There is nice shape and not so nice shape. A nice shaped but well performing stone would be better to me than a top performer that is wrongly shaped. A nice pear shape is just something most people would know when looking at a drawing, not of diamonds but just a drawing of a pear outline. Before cut grade was promoted it was after all shape.

I remember reading an article years ago about round brilliant diamonds where it was stated that the American cut was just like the top of a pencil and the cut in Europe was more spread and what people over there preferred and also looked larger, I take it this was the 60/60 or maybe the table was even larger I don't know.

I have noticed sometimes when looking at oval cuts, in sapphire, that sometimes there is a pavilion bulge that is quite ugly.

I remember someone writing on pricescope before who was quite famous in the diamond world and he had a system like Oldminer's too but it was all about facet symmetry and grading stones on that, not about hearts & arrows but he was doing very minute calculations on the facets and meant to say it did make a difference in the quality of cut.

I for one am very interested to learn more about this aspect of shape.

I notice Oldminer said his perspective was on near colorless diamonds but had mentioned fancy color diamonds beforehand. Is Colorless diamonds not included in the cut shape parameters?
 

Karl_K

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Oldminer|1478628334|4095264 said:
Over depth decreases visual size.
Bangs head on wall again.
In fancy cuts depth does not equal spread.
A deep stone can and does in some cases have better spread than a much shallower stone.
A while back on here 4-5 asschers were being compared and the depths ranged from 65% to 75%.
The 75% depth stone had the largest spread of the group.
It was both physically and visually the largest of the bunch.

I agree about care being needed not to get overly thin girdles.
 

Karl_K

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Pyramid|1478633983|4095311 said:
I have noticed sometimes when looking at oval cuts, in sapphire, that sometimes there is a pavilion bulge that is quite ugly.
They are cut that way in diamond also, and for the same reason, keeping weight.
 

Karl_K

Super_Ideal_Rock
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Oldminer|1478628334|4095264 said:
Anyone who would come forward and say the only consideration in "cut quality" is light behavior and light return" would be dead wrong.
I would rank patterns right up there near the top so that would not be me.
That includes the outline of the stone.
Getting good patterns and high light return is very hard, light return or patterns is fairly easy in comparison by themselves.

What is the proper outline?
You opinion may be vastly different than mine.
I prefer the looks of wide short pears, others may like long skinny ones, some may like them in between.
Who is right? Who has the authority to say one outline is better than the other?
 

Karl_K

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crocop|1478622274|4095214 said:
Karl_K|1478618098|4095181 said:
while you are leading people to my thread, can someone please comment on my last post!

Thanks :dance:
Forum rules prohibit me from commenting on specific stones.
This is my opinion not a fact.
In general I look for lots of red and green with no large white areas and maybe just a hint of blue here and there in pears.
Near the tip your going to get lots of small areas of red,green,white mixed together.
That's normal but more red and green than white is good.
 
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