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GIA Ex Cut Grade but bad HCA???

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M

Megenoita

Rough_Rock
Joined
Apr 22, 2008
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36
1.01 F VS2 (GIA)

Depth 61.8
Table 57
Pav. angle 41.2
Crown angle 35
Crown heigh % 15
Pav. depth % 44

The stone only receives a 3.6 on the HCA, but Excellent from GIA. I want to buy a stone that is a top cut grade, and just want to make sure that I should trust GIA over the HCA--am I right?

From what I''ve been reading, GIA''s cut grade is very much based on the human eye, and not just numbers. So I can be safe trusting that GIA got it right (?). I don''t need a superideal here, just a true Ideal cut in terms of light performance.

Thanks for the help!
M
 
It might be a borderline case, where you can''t really tell the difference between this and a "true" ideal (according to PS standards,) or it could be a case where you can tell the difference.

It''s not that you should trust GIA over HCA. Perhaps the safer route is to find a stone in the intersection of {GIA EX} and {HCA 0-2}.
 
Date: 1/27/2009 5:26:18 PM
Author:Megenoita
Depth 61.8
Table 57
Pav. angle 41.2
Crown angle 35
Click to expand...

Tossed out the Pavilion Depth and Crown Height percent measurements because those are estimates based on the crown and pavilion angle measurements which are actually measured by the computerized proportions analysis machines...

Generally speaking, we would avoid the combination of a steep crown and pavilion angle because it will reduce visual performance and assume that the HCA resuls are based on that premise. Would prefer a crown angle between 34.3 - 34.8 degrees offset by a pavilion angle between 40.6 - 40.9 degrees (personally). However other combinations of crown and pavilion angle will produce similar visual performance - such as a shallow crown angle offset by the steeper pavilion angle -or- a shallow pavilion angle offset by the steeper crown angle. Play with the HCA a bit and see what happens, drop the pavilion angle to 40.5 degrees and the HCA rating improves dramatically, the results get a "little" better if you decrease the crown angle significantly (try 33.0 degrees). Now ask yourself "why?"

The pavilion angle is the primary reflective surface of the diamond design that reflects light back up towards the viewer, thus the angle of the pavilion facets is critical to visual performance - if you''ve got to bend a parameter, better to bend the crown angle and keep the pavilion angle within the tighter range.
 
Thanks for the responses, everyone. I understand the preferences for pavilion angles/crown angles, and I have played with the numbers to see what would score better with the HCA. My primary concern is knowing how much to trust the GIA Excellent cut grade. I am going to go read those articles linked, thanks!

M
 
Date: 1/27/2009 7:44:43 PM
Author: Megenoita
. My primary concern is knowing how much to trust the GIA Excellent cut grade. I am going to go read those articles linked, thanks!
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Megenoita,

Stone''s the man, as we are backing each other up here, proving you some textual cause to give pause to GIA.

But, you''re smart, too, to give pause to info that would contradict a major force in diamond engineering.

I say....just don''t place your wager with either. I try to say it here, too.

Basically, a lot of diamonds comport to BOTH excellent for GIA, and HCA, too. Why fret. Go with one that coordinates with both. And then (if you''re willing to ignore the ADDITIONAL evidence that smart people here seem to be seeking to authenticate good cut quality, found with reflector technology confirmation, or an AGS stand in for it) have a nice day!
 
I''m going to post the specs of the stone I picked finally. I found a stone with a great HCA and Ex GIA Cut Grade, but it was over my budget by too much. I chose a stone that was a GIA Ex/Ex/Ex, but I didn''t see the HCA numbers until now. HCA score is 4.6 = Good, only if price is your main criterion.

Depth 62.4
Table 58
CA 35.5
PA 41.2

I read some of the articles, and I plan to read all of them when I have time, but for now, I just feel that it is safe to trust GIA. I know that in the GIA tests, they do have visual assessments, whereas the HCA is 4 numbers. And I remember reading Jan''s link:

http://dbof.com/why-the-hca-does-not-work.htm

about why the HCA isn''t foolproof. I understand that the GIA method is not a science, but I have to have faith that a diamond that is an Ex/Ex/Ex with the new cut grade system MUST be a top performer. Maybe not a superideal, but certainly a true ideal and more importantly, a diamond that is going to have impressive light performance. These are my thoughts as of now. I''ll report back if the stone is a dud!

Thanks,
M
 
M,

Since you''ve made your decision...look up Michael Cowing...and see if you can get a hold of him. You do need to update your references. DBOF''s point of view, tautologically, can''t help you. They say HCA doesn''t work, only because it hadn''t seen the diamond. But, that logic would need to say the same thing about GIA''s system...meaning, if you DON''T believe HCA, neither can you believe GIA.

But, talk with Michael. He might figure out a way to make you feel better.

Or, you could go mope somewhere together.
 
Every place where advice and instruction is dispensed, there is a "party line." If I went to culinary school, there is the official way to hold a knife, there is the official way to make beef stock.

If I went to an online knife forum, there is a certain (considered best) way to sharpen a knife of very hard (like VG-10) steel.

If I went to a coffee and espresso forum, the advice would be "the grinder is at least as important as your machine, if not more important," or "15-18 grams of finely ground coffee pulled for approximately 25 seconds at 200 degrees F."

If someone wants to be happy in their environment, it is best to not go against the party line. And here, HCA is the original founder of the party. It's actually very fair, you know...0-2 should be considered, 2-3 is still probably worth considering if your budget is tight, and so on. So, either you find a place that shares your philosophy, or you share the philosphy of the place.
2.gif
 
Reg Guy,

DBOF states many reasons why they feel the HCA doesn''t work, not just that the diamond isn''t viewed in order to assess its score. But with the GIA Cut Grade, the diamond IS viewed to assess the score. Your own words seemed to have caused you to slip up, because you said, "With the HCA, you don''t see the diamond, and with GIA, you don''t see the diamond", but with the HCA, NO ONE sees the diamond, of particular importance being the HCA scoring system, and with GIA, the "scorers" DO see the diamond.

I agree that DBOF''s arguments seem to be a tautology, and it''s admittedly very redundant. But the obvious truth about the HCA is that it takes 4 measurements, and 50% are not precise. GIA''s system takes into account so many more factors, INCLUDING seeing the stone. So although the GIA system has its issues, I think it should be trusted over punching 4 numbers into a calculator--is that not obvious to everyone else?

JulieN, thanks for pointing that out--I forgot that this forum is probably more inclined to love the HCA! I myself do like it, but with the new GIA cut grade system, I think it must be favorable to the simplicity of the HCA.

M
 
Date: 1/29/2009 4:40:16 PM
Author: Megenoita
Reg Guy,

DBOF states many reasons why they feel the HCA doesn't work, not just that the diamond isn't viewed in order to assess its score. But with the GIA Cut Grade, the diamond IS viewed to assess the score. Your own words seemed to have caused you to slip up, because you said, 'With the HCA, you don't see the diamond, and with GIA, you don't see the diamond', but with the HCA, NO ONE sees the diamond, of particular importance being the HCA scoring system, and with GIA, the 'scorers' DO see the diamond.

I agree that DBOF's arguments seem to be a tautology, and it's admittedly very redundant. But the obvious truth about the HCA is that it takes 4 measurements, and 50% are not precise. GIA's system takes into account so many more factors, INCLUDING seeing the stone. So although the GIA system has its issues, I think it should be trusted over punching 4 numbers into a calculator--is that not obvious to everyone else?

JulieN, thanks for pointing that out--I forgot that this forum is probably more inclined to love the HCA! I myself do like it, but with the new GIA cut grade system, I think it must be favorable to the simplicity of the HCA.

M
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People give a stone the color and clarity grading at GIA. But the cut grade of an individual stone is not determined by people. So, in effect, no one sees it. (RG is correct.)
 
Hey, M, I could be wrong. But, we may be confusing procedures with criteria. Yes, GIA must see the stone, but that's just the procedure...and it's their system for getting paid. Another problem with their procedure, is that they require that whatever they actually measure, they round...another problem. But...you're issue is I think criteria.

I think whether seen by GIA or not, the criteria they use to judge cut is abstracted from numbers, ostensible measurements...whether the diamond is in hand or not. In these ways, the two systems are the same.

And...I think it is this approach that DBOF criticises, no?

(edited to add...)...I was serious about Cowing, though. He'll back their analysis. I would take his research seriously.

(edited to add, again)...thanks, Julie...
 
Date: 1/29/2009 4:46:11 PM
Author: JulieN

Date: 1/29/2009 4:40:16 PM
Author: Megenoita
Reg Guy,

DBOF states many reasons why they feel the HCA doesn''t work, not just that the diamond isn''t viewed in order to assess its score. But with the GIA Cut Grade, the diamond IS viewed to assess the score. Your own words seemed to have caused you to slip up, because you said, ''With the HCA, you don''t see the diamond, and with GIA, you don''t see the diamond'', but with the HCA, NO ONE sees the diamond, of particular importance being the HCA scoring system, and with GIA, the ''scorers'' DO see the diamond.

I agree that DBOF''s arguments seem to be a tautology, and it''s admittedly very redundant. But the obvious truth about the HCA is that it takes 4 measurements, and 50% are not precise. GIA''s system takes into account so many more factors, INCLUDING seeing the stone. So although the GIA system has its issues, I think it should be trusted over punching 4 numbers into a calculator--is that not obvious to everyone else?

JulieN, thanks for pointing that out--I forgot that this forum is probably more inclined to love the HCA! I myself do like it, but with the new GIA cut grade system, I think it must be favorable to the simplicity of the HCA.

M
Click to expand...
People give a stone the color and clarity grading at GIA. But the cut grade of an individual stone is not determined by people. So, in effect, no one sees it. (RG is correct.)
Click to expand...
Ditto Julie, I agree Ira is correct.
 
Ira,

Does Cowing have anything up online (website, tutorial etc.)??

Thanks!
 
Just a suggestion about selection here:
I am a believer in GIA, and their cut grade- BUT- having hung around here for only a few days, I hear a lot of talk about EX cut grade stones that are "Steep/Deep"
Without getting into the whole conversation, I do have an approach that might benefit you.
Stay away from stones with depths over 62%.
My personal preference would be no deeper than 60%- and with a slightly larger table.
If you were shopping a 5ct D/VVS1 you''d have little choice. But shopping for a one carat F/VS2 allows you literally thousands of choices.
Why not stay away from any that are deep?
 
Date: 1/29/2009 5:09:05 PM
Author: Judah Gutwein
Ira,

Does Cowing have anything up online (website, tutorial etc.)??

Thanks!
Click to expand...
I''m sure Michael would be pleased to get your review...but probably his most heartfelt ideas are in context...providing contrast...found right here in his comments in Pricescope''s threads....
 
Date: 1/27/2009 5:26:18 PM
Author:Megenoita

The stone only receives a 3.6 on the HCA, but Excellent from GIA. I want to buy a stone that is a top cut grade, and just want to make sure that I should trust GIA over the HCA--am I right?
Click to expand...
They are two different tools. The HCA is blind and intended for rejection (not selection). The GIA grade is assigned to the actual diamond.

From what I've been reading, GIA's cut grade is very much based on the human eye, and not just numbers. So I can be safe trusting that GIA got it right (?).
Click to expand...
In many cases yes. The proportions grade is not assigned by a human observer though. The basic measurements are compared to their database (developed using human observation studies) and a corresponding grade is assigned. While the center of their system agrees with other popular and successful metrics there is a steep/deep range which GIA allowed into the top grade that many experts take issue with. The problem with diamonds at the borderlines is that the GIA system is not truly diamond-specific. The numbers are averaged and rounded: CA, for example, is one average number which stands for eight different measurements. Same with PA, stars and lower halves. This means there is no accounting for variance from that average number which could create leakage zones in one diamond...but not in another with the same average numbers & grade.

When the diamond's measurements fall outside the overlapping top grades of successful systems I encourage getting more information.

I don't need a superideal here, just a true Ideal cut in terms of light performance.
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FYI, the AGS cut guidelines predict this combo (57/41.2/35.0) as AGS3 in light performance.
 
I want to clarify on the GIA cut grading...so it is NOT at all based on gemologists'' looking at the stone under various lighting such as the GIA Diamond Doc??? It is ONLY a computation, that''s it?

Thanks,
M
 
Date: 1/29/2009 6:30:44 PM
Author: Megenoita
I want to clarify on the GIA cut grading...so it is NOT at all based on gemologists' looking at the stone under various lighting such as the GIA Diamond Doc??? It is ONLY a computation, that's it?

Thanks,
M
Click to expand...
M

Though that IS what we've said...

a) consider if some new person saying it will make you believe it
b) more importantly...consider that YOUR criteria could be upgraded. That is...you know...there is a reason that they call it "cut." It's certainly impressive that based on an agreement of really all the major grading agencies, that based on how a diamond's angles are measured, they are willing to assign a cut grade. This is substantially true of the most rigorous grading agency as well...AGS. They abstract and assign a grade from the measurements...except they don't round, and they assess what is specifically in hand. Nevertheless, they, too essentially then abstract that data, and assign a grade, virtually ignoring what the diamond they just specifically measured is actually "doing."
c) so, though no agency assigns a cut grade based on some direct measure of performance, some EGL may enhance their report with imagem valuation...but I don't think it establishes a grade...I really don't know...and I've never read anything very clear here about how EGL assigns anything.

So...I'd encourage you to give up one prejudice, and trade it in for another. Like I've said. The merit you've assigned to GIA hasn't gone away, just because you've misunderstood it. All their clinical studies were really done, they did gather that data, and based on it, assigned categories and hierarchies to cut classes, producing the excellent cut grade your diamond has been assigned. Presumably, AGS gives a 3, in a 10 level scale, to what GIA gives it's number one spot...on a 5 level scale...where the bottom level is never used, and I don't think I've ever seen the 4th level used either. HCA just thinks it's fowl. But...GIA really does like it...you have read that correctly. The question is...do you trust the criteria they applied. Previous articles Stone pointed you to get into why you might or might not, taking a critical view of it. If you abstract from Cowing, he'll hand wave as to why you might trust them...but be careful about asking him tough questions, like what about IS analysis, and stuff like that.
 
Date: 1/29/2009 6:30:44 PM
Author: Megenoita
I want to clarify on the GIA cut grading...so it is NOT at all based on gemologists'' looking at the stone under various lighting such as the GIA Diamond Doc??? It is ONLY a computation, that''s it?

Thanks,
M
Click to expand...
Succinctly, yes.

Given the minute aspects being measured, the lab locations assign the proportions grade based on scanner results for sake of repeatability. Finish grades (and color/clarity of course) are still done by visual examination.

Interestingly, in our GIA coursework we''re trained to visually assess brightness, fire and scintillation (sparkle and pattern) using diffuse and direct lighting of the kind you see in DiamondDock. In addition to hand-measured proportions those qualities appear on the GIA diamond grading form a GG might use to assign a cut grade away from the lab without benefit of a scanner. This is logical, since hand-measured angles are tenuous at best.
 
Regular Guy,

Do you usually talk to people you don''t know this way:

"a) consider if some new person saying it will make you believe it"

"So...I''d encourage you to give up one prejudice..."

I wonder if you consider the irony of the prejudiced assumptions you needed to make in order to imply the insults that you conveyed with those two statements.

Anyway, my only prejudice is trusting the visual...do you disagree that seeing is the best way to determine a diamond''s beauty?

In GoodOldGold''s video on the new GIA Cut Grade system, it is presented that the cut grading is based on brightness, fire, scintillation, weight ratio, proportions...but really what he meant was that the system''s development, not the resultant grading, was based on these components--the presentation confused me. GIA''s site can be read the same kind of way if you saw GoG''s first. Then, with the links in this thread, the GIA Diamond Dock was closely examined, and I continued to believe that they were criticizing the grading, not the system''s foundation only. Lastly, there was a PS thread where Rhino talked about the grading system and you could read it to mean that this is how GIA grades. I''m sorry for being confused, but it is not that I am ignoring anyone; I''m trying to find the truth when I thought I was hearing conflicting views. I have no preference for what method(s) is/are the best.

I have to say that I am very disappointed that there is no lab that grades based on direct visual assessment!

I get the diamond in a couple days...I will report back if it is a dud or stud!

M
 
Date: 1/31/2009 7:55:36 AM
Author: Megenoita

I get the diamond in a couple days...I will report back if it is a dud or stud!

M
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Have you viewed enough diamonds to know the difference? I ask that sincerely.
1.gif


Almost all diamonds will look good in some lighting situations, but only some diamonds will look good in all lighting situations. The cut on the one you have is not conducive to the latter.
2.gif
 
Ellen,

I''ve viewed thousands of diamonds, but I am still no expert.

I received the stone and I feel that this diamond I ordered was more of a dud than a stud
7.gif
.
 
Date: 2/4/2009 1:53:20 PM
Author: Megenoita
Ellen,

I''ve viewed thousands of diamonds, but I am still no expert.

I received the stone and I feel that this diamond I ordered was more of a dud than a stud
7.gif
.
Click to expand...
Sorry to hear that, you will find the right one!
 
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