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Help! GIA Excellent Cut Vs. HCA

F

Fife

Rough_Rock
Joined
Jul 9, 2012
Messages
64
Here's the original post. [URL='https://www.pricescope.com/community/threads/just-paid-for-this-diamond-good-value-1-53-h-vs1.177565/#post-3232481']https://www.pricescope.com/community/threads/just-paid-for-this-diamond-good-value-1-53-h-vs1.177565/#post-3232481[/URL]

I had already purchased the diamond before learning about HCA.

GIA rates this diamond's cut in its highest tier - Excellent.

A few other forum folks pointed out that this diamond fails the HCA test. If this diamond does so poorly on the HCA test, why would GIA give it the highest cut rating?

ring_68.jpg

Carat: 1.53
Cut: Excellent
Lab: GIA
Certificate: 2141708547
Clarity: VS1
Color: H
Polish: Excellent
Symmetry: Excellent
Fluorescence: None
Table: 57
Depth: 62.4
Length: 7.35
Width: 7.38
Height: 4.6
Culet: None
Girdle: Thin to Medium
 
If you read in your original thread, Christina answered that question for you.

Briefly, GIA Excellent cut score is a BROAD, generous range. Thus, things that are not ideally cut, if numbers and combinations of the numbers fall within a certain range, will be given a cut score of excellent.

here's some fun reading for you
http://goodoldgold.com/Articles/GIAExConsumersBeAware/
 
My Diamond got a 5.5 on the HCA. That's a really terrible score. A mega-failure score. Basically this diamond is going to look like garbage. Are you saying that GIA's Cut rating is so broad that it encompasses terrible cuts all the way through superb cuts?


There's got to be more to this.
 
Fife|1342045477|3232553 said:
My Diamond got a 5.5 on the HCA. That's a really terrible score. A mega-failure score. Basically this diamond is going to look like garbage. Are you saying that GIA's Cut rating is so broad that it encompasses terrible cuts all the way through superb cuts?

There's got to be more to this.
Click to expand...


You are assuming that any one tool is capable of choosing what you think is the best of the best. Unless you yourself make the tool this will obviously never be the case. HCA isn't foolproof - there will be some beautiful stones that score poorly and some wretched stones will score well, it doesn't take into account all the proportions that fine-tune a stone, and numbers on the GIA report are averaged around eight sections and then rounded to begin with.

GIA and HCA (and AGSL and AGA and everything else) were developed at different times by different people/corporations with different interests and different preferences for different purposes... they'll have some overlap, of course, but I don't recommend blind faith in any of them - your final choice(s) should be evaluated on its own merits.

I do not think that diamond will look like garbage. I think pav &/or crown may have been rounded up, there is some leakage (light escaping out the back through the pav facets) under the table but the diamond is also slightly tilted in such a way as to exaggerate that area of leakage. I do think it'll be visible IRL if you look closely but I do not think it will be obvious or obviously detrimental... but there are definitely better choices.
 
I also understand GIA has a range of percentages of excellent cut than AGS. HOWEVER, table percent of 56%, depth of 60-61%, crown angle of 34 degrees and pavilion angle of 41.2 degrees is bulls eye target excellent for GIA. AGS will also rate this stone AGS 000 with their bulls eye target being more 57% table, 34 crown angle and 41 degree pavilion.

In addition another PS link looked at light leakage on crown 34degrees and 41pavilion angles and found no light leakage (table 56%), he found the same with 34 crown 41.2 pavilion (no light leakage).

Why then on the HCA if you change the pavilion from 41 to 41.2 that the stone goes from ex ex ex ex 1.5 to vg vg vg vg 3.0

After all the data from GIA and AGS show 41 pavilion ideal WHY does HCA consider 40.5 pavilion better?

There is a paper on GIA, AGS their overlapping areas of ideal/ex cut and the towolsky (cant spell) formula etc, and the intersect is 41 degreees not 40.5 as the HCA recommends.

A stone that fits both AGS ideal, GIA ex you would think should pass HCA and in this example 34 degree crown and 41.2 degree pavilion it does not pass HCA by 0.2 on the pavilion. As the GIA bulls eye gold standard is a 41.2 pavilion and 34 crown HCA would knock out these diamonds.

Why is the steep pavilion angle cut off in HCA discrediting stones that have been analyzed and studied cut with optimal cut, and dealt with by stone cutters with so many years of history?

I dont think HCA can be all that correct.
 
2023|1342046892|3232561 said:
I also understand GIA has a range of percentages of excellent cut than AGS. HOWEVER, table percent of 56%, depth of 60-61%, crown angle of 54 degrees and pavilion angle of 41.2 degrees is bulls eye target excellent for GIA. AGS will also rate this stone AGS 000 with their bulls eye target being more 57% table, 34 crown angle and 41 degree pavilion.

What makes you say that? Where are the charts/data you are using coming from? GIA is proportions-based - they provide a tool that considers and grades a variety of crown and pavilion (and other) proportions for a given table size. GIA is suggesting a range of discrete cut grades, they are not suggesting that you take the middle of any given range and call it the bulls-eye...

In addition another PS link looked at light leakage on crown 34degrees and 41pavilion angles and found no light leakage (table 56%), he found the same with 34 crown 41.2 pavilion (no light leakage).

Yes, lots of factors here. Vision. Eye for details like this (someone who has spent a lifetime looking at diamonds will of course be more likely to spot nuances). Rounding and averaging that led to those printed numbers. Optical symmetry. This is why we recommend always looking at the stone rather than buying strictly by numbers.

Why then on the HCA if you change the pavilion from 41 to 41.2 that the stone goes from ex ex ex ex 1.5 to vg vg vg vg 3.0
Because those are the guidelines the creator of the tool decided on

After all the data from GIA and AGS show 41 pavilion ideal WHY does HCA consider 40.5 pavilion better?
It does not make any such blanket statements - hence the divisions into BIC/TIC/FIC as well as numerical score

There is a paper on GIA, AGS their overlapping areas of ideal/ex cut and the towolsky (cant spell) formula etc, and the intersect is 41 degreees not 40.5 as the HCA recommends.
Again, in what context are you considering the 40.5 specifically? Without any context this statement does not make any sense

A stone that fits both AGS ideal, GIA ex you would think should pass HCA and in this example 34 degree crown and 41.2 degree pavilion it does not pass HCA by 0.2 on the pavilion. As the GIA bulls eye gold standard is a 41.2 pavilion and 34 crown HCA would knock out these diamonds.
HCA is known to penalize higher pav angles

Why is the steep pavilion angle cut off in HCA discrediting stones that have been analyzed and studied cut with optimal cut, and dealt with by stone cutters with so many years of history?

I dont think HCA can be all that correct.
I would most certainly consider Garry an expert. Absolutely no question.

HCA is simply a tool. With uses and limitations. It can help you decide whether a stone is likely worth further investigation... but noone on here is suggesting that you live or die (or buy) by it and it only.
Click to expand...
 
I posted the paper: shows graphs etc

AGS (Ideal) Crown Angle: 33.7° - 35.8° Pavilion angle: 40.2° - 41.2° Table Size: 52.4 - 57.5%
GIA (Excellent) Crown Angle: 31.5° - 36° Pavilion angle: 40.6° - 41.8° Table Size: 52 - 62%

Summary, as said yes I believe that GIA is broader, however correlating the overlaps and the different parameters as in the link I posted explains it better. On pavilion HCA heavily favours AGS...

40.5 is from the HCA calculator if you keep all things consistent, but go from 41 to 41.2 degrees on pavilion angle it changes the outcome. simoly saying 40.5 is the sweet spot for HCA by using and looking at the calculator.
 
In the paper figure 5 shows the bulls eye target for GIA being 34 crown 41.2 pavilion, figure 6 shows AGS 34 and 41 and figure 6 shows where GIA and AGS overlap. Given the tables are the same. On the HCA it makes no sense if all else is constant to change from 41 to 41.2 on pavilion from a 1.5 to a 3.0. It definitely does seem HCA is biased towards AGS.
 
2023|1342048533|3232571 said:
I posted the paper: shows graphs etc

AGS (Ideal) Crown Angle: 33.7° - 35.8° Pavilion angle: 40.2° - 41.2° Table Size: 52.4 - 57.5%
GIA (Excellent) Crown Angle: 31.5° - 36° Pavilion angle: 40.6° - 41.8° Table Size: 52 - 62%

Summary, as said yes I believe that GIA is broader, however correlating the overlaps and the different parameters as in the link I posted explains it better. On pavilion HCA heavily favours AGS...

40.5 is from the HCA calculator if you keep all things consistent, but go from 41 to 41.2 degrees on pavilion angle it changes the outcome. simoly saying 40.5 is the sweet spot for HCA by using and looking at the calculator.
Click to expand...

As Yssie already stated, the HCA does seem to penalize stones with a PA over 41, but I wouldn't agree that it heavily favors 40.5. HCA only calculates based on 4 measurements, thats why you can't live or die by it. GIA also rounds its angles on the report, so you can't depend on those numbers either. Yssie can tell you exactly by how much and where they round, I don't know the details. But you also need to realize that a difference of .2 either way can make a significant impact on the way a diamond looks and performs. Without a sarin report of the stone you don't know which way the numbers were rounded. I'm not saying that your diamond is going to be a dog. I think that there will be leakage under the table, but Yssie pointed out that you may not even see this. I don't think you should panic at this point. You've already purchased the diamond, take a look at it in many different lighting environments and see what you think IRL. You have a return policy so no worries. :)) If you love it, you keep it, if you don't then we can help you if you'd like. Did you have the diamond sent loose or is it set?

I just saw your last post and wanted to comment, I'm sure someone else can explain it better, but it's not just the PA that you need to consider, you need to consider how the PA correlates with the CA. Your stone has a steep CA, typically steeper CA perform better when they are coupled with a shallower PA. You really need to consider all the proportions as a whole and not base it on one or two. The HCA very well may favor AGS cut standards but again, it's because they are much more narrow than GIAs. It's thought that GIA buckled to pressure from cutters to include some criteria into their excellent cut grade, mainly so that the cutters could retain more ct weight and thus make more for their stones while still receiving an EX cut grade....or so I understand it. Others will chime in. Maybe even Garry himself. :))
 
http://www.acagemlab.com/news/JoG07305.pdf

Hi Christina I am posting this article again. It clearly show what I am trying to say. The sweet spot or GIA is crown 34 pav 41.2 the sweet spot for AGS is 34 crown 41 pav given table and depth is optimum. Im not the original poster it is not my stone, I am pointing out it makes no sense HCA would discredit a stone from 41 to 41.2 pav given other parameters are optimum unless it is biased against GIA, as 34 crown and 41.2 is considered the absolute sweet spot for GIA.

I hope someone looks at the paper and understands me where I say 34 crown and 41 angle by both AGS and GIA are ideal and ex, so 41 being the cut off on HCA being all else is optimum makes no sense. It seems 40.5 is more the desired target.

Ps i know GIA rounds odds to even degrees etc that is not my point.
 
No single proportion can define a diamond, it must be taken as a whole. However, the pavilion angle is what many consider the most critical element.

In the most popular metrics discussed here: AGSL, GIA, HCA and AGA - average pavilion angles from 40.6 - 41.0 have the greatest approval cross-system. There are exceptions but that range is highly considered the safe-zone according to those metrics, as well as the consensus of the most high-achieving diamond cutters and research gemologists I know. For the record, the AGSL shows most 0 light performance candidates to be in that range. The HCA and AGA prefer the shallow side of that range (and a bit lower). GIA prefers the steeper side of that range (and a bit higher). This is why the GIA gives EX to steep diamonds the HCA rejects and the HCA gives thumbs-up to shallow diamonds the GIA system doesn't reward.

The reason I focus on the pavilion angle is because those facets are the engines driving light return. The table and crown angle are also important but all optics in a diamond begin with light returning from the pavilion. To me, a precisely cut stone that has 41.2 paired with 34.0 can be terrific and bright (with an appropriate table size and break facets) but the lower crown height resulting from the shallower CA will not produce the robust dispersion of a crown between 34.5-35.0 with a Tolkowsky table and a 40.8 PA. No worries; tastes differ and some people clearly prefer the "brighter" look of such a configuration in the same way some prefer 60/60 makes. Others like the look of a diamond with balanced brightness and dispersion.

As it relates to metric-approval, something which has not been discussed here is cut-consistency. Remember that any given pavilion angle on a report is a number which has been averaged across eight pavilion facets. Taking a "normal" Tolk table and crown as a given; even if a diamond with a 40.8 pavilion is not precisely cut the facets which deviate from average can withstand several tenths of deviation before risking any loss of light in the return. But if you stray to the (borderline-steep) 41+ area or the (borderline-shallow) 40.6- area --- again, with "normal" table and crown --- there is far more risk that some of the pavilion facets could fail to partner in a way that results in the critical angle required for most robust light return.

Some past discussion here:
https://www.pricescope.com/community/threads/gia-excellent-cut-cut-advisor-5-8-what-to-do.87439/
 
I hope that someone else can respond to your question who is better qualified than me but I do think that I understand what you are saying. I believe that the two stones in your example are getting a much different HCA score is because a .2 difference really does make a significant impact on the performance of the stone. If you play with the HCA for a bit and enter a bunch of different stones and then change the CA or PA by .1 or .2 .3 etc, then you can see how if affects the outcome. I think that you understand this. What I think you might be missing is that a 41 or 41.2 PA may be fine when it has a 34 or 34.5 CA, but coupled with the steep crown of your stone 36CA, the proportions don't work as well. If your stone had a CA of 33 and the PA remained at 41.2 the HCA would result in a score of 2. So, as you can see ALL the proportions have to work together. I do agree though, that the HCA does grade harshly on stones with steeper PA's and I don't have enough experience to say whether it's appropriate or not.

There was an article done by Jonathon, I mentioned in your last thread and I haven't located it yet, but he did a study with a stone that GIA considered an EX cut, however the stone failed all the test that it was given. So based on that info the stone should have been a dog. He then coupled the 'dog' with an AGS000 HA ideal cut diamond and presented the two to several of his customers and asked them which stone that found more visually appealing. I cant remember how many he asked but I do remember that all but one chose the 'dog' as the more beautiful of the two diamonds. So, while all of these tests are helpful in weeding out a long list of online inventory, they aren't infallible and in the end your eyes will tell you if the diamond is a beauty or not. I really hope that someone will come along and be able to answer all your questions to your satisfaction, unfortunately I just don't have enough experience to explain the hows and whys. ;)) But, really I wouldn't get too stressed out, like I said before, you are working with a reputable vendor and have a good return poliy, you will know when you see it if it's the one. :))


I see that John just posted, so most of what I said is irrelevant now, he's the real expert! ;)
 
The article:

1. The Table% chart indicates how many combinations of c/p angles may earn the top cut grade per table percent value. A table of 56%, according to that graph, permits the most top-cut-grade c/p combinations. The chart by itself does not indicate that a table of 56 is a visual optimum, merely that a cutter who aims for a 56 table will, all else equal, have the most flexibility wrt c/p combos.

2. The second figure, figure 5, is GIA cut grade chart for 56 table. EX c/p combos are outlined in red, and the author notes that the center of this EX outline is 34/41.2. Likewise figure 6 indicates that the middle of the AGSL 0/1 outline is 33.75/41.1 - but the 0 & 1 ranges are very skewed on that chart.

3. If I am understanding correctly the author, in the summary page, asserts that the "best" proportions can be derived from looking at what's in the middle of the outlines of the two labs' top cut grades. I could not disagree more, and in fact this is precisely what the labs themselves are trying to avoid:
A) The purpose of permitting a range of proportions combinations in the top cut grade(s) is to avoid pigeon-holing to a point-source ideal, and to account for the fact that people may find beauty in different flavours of light return, assuming a certain quantity of light return.
B) A cutter has a definite advantage in aiming for that "sweet spot centre" - best odds of cutting a winner, since there is some flexibility on the proportions combos & some margin for adjustment. To suggest that the "sweet spot center" describes the most visually appealing stone simply *because* it is the centre is, in my opinion, completely misguided.


Updated GIA charts http://www.gia.edu/diamondcut/pdf/booklet_cut_estimation_tables_lowres.pdf
From this page http://www.gia.edu/diamondcut/08_tools_for_the_trade.html
 
"In the most popular metrics discussed here: AGSL, GIA, HCA and AGA - average pavilion angles from 40.6 - 41.0 have the greatest approval cross-system. There are exceptions but that range is highly considered the safe-zone according to those metrics, as well as the consensus of the most high-achieving diamond cutters and research gemologists I know. For the record, the AGSL shows most 0 light performance candidates to be in that range. The HCA and AGA prefer the shallow side of that range (and a bit lower). GIA prefers the steeper side of that range (and a bit higher). This is why the GIA gives EX to steep diamonds the HCA rejects and the HCA gives thumbs-up to shallow diamonds the GIA system doesn't reward."

EXACTLY my point.

If you put into the HCA a pavilion score lower than this range you have just quoted as ideal 40.6 - 41, just using 34 crown and 40.4 (0.2 off) for the pavilion the HCA still gives it a score of EX EX EX EX and 0.6 on the output.

That is outside what all experts consider optimum yet HCA considers it amazing.

If GIA believes 41.2 is optimum and is its 'target' figure it seems unfair to discredit GIA and its millions of diamonds that it cuts, analyzes and looks at, yet allow a 40.4 that falls 0.2 below what AGSL, GIA, and AGA average as ideal as an excellent score of 0.6 - which is below what all labs to be 'subpar'.

HCA is on online tool, and seems to favour shallower pavilions over a lab GIA that has years of diamonds cutting and expertise.
 
Yssie, if you disagree with pidgeonholing, then why would people encourage people to discount stones based on HCA scores, stones that as I said can fall within the margins of what both AGS and GIA considered Ideal and Excellent respectively. As the example I said 34 crown 41.2 pav (what GIA considers their bulls eye target) is outside HCA.

Christina I completely agree a crown of 36 vs a pav of 41 would not be as precise a cut. What I was discussing was the target range for GIA, and what their goal is. I know also GIA and AGS have different parameters, etc, but HCA does discredit what GIA gives its absolute best proportion 34 crown and 41.2 pavilion.

Yet HCA allows absolute winners that fall below what ALL reputable labs believe is outside the safe zone, a pav angle at 40.4 with a crown of 34.

Just think the Pav angles are a bit strange in the HCA
 
2023|1342059674|3232662 said:
Why disagree with what people have 150 years of cutting experience in?

Well we're all allowed to think for ourselves on here ::)

And if you disagree you said

"The purpose of permitting a range of proportions combinations in the top cut grade(s) is to avoid pigeon-holing to a point-source ideal, and to account for the fact that people may find beauty in different flavours of light return, assuming a certain quantity of light return."

then why would people automatically discount stones based on HCA scores, which fall within the margins of what both AGS and GIA considered Ideal and Excellent respectively because it falls above 2. As the example I said 34 crown 41.2 pav (what GIA considers their target).

Oh I left out that part of my post when I copied it in from notepad - my personal HCA cutoff would be more like 3, even 3.5, not 2 - which would allow for some of those 41.2+ combos

Christina I completely agree a crown of 36 vs a pav of 41 would not be as precise a cut. What I was discussing was the target range for GIA, and what their goal is. I know also GIA and AGS have different parameters, etc, but HCA does discount what GIA gives its absolute best proportion 34 crown and 41.2 pavilion.

Yet HCA allows absolute winners that fall below what ALL reputable labs believe is outside the safe zone, a pav angle at 40.4 with a crown of 34.

I guess I just don't understand what the issue here is. Yes, HCA is neither fool-proof nor folly... but that's what we're saying too.
Click to expand...
 
I just get worried when people discredit stones based on HCA, when I know for a fact, that a 57% table, a 34 crown and a 41.2 pavilion falls directly in the centre of what GIA considers excellent (perfectly well well well within the range, right in the centre) and is also supported by the link Yssie attached.

But when you use the HCA, this pops out at a 3.

Currently the HCA pavilion range, discredits GIA bulls eye excellent cuts, BUT pavilions that fall below 40.6 (which apparently ALL labs agree is outside the range) at 40.4 an HCA pops out a 0.6.

I would think that output for pavilion angles on the HCA should be higher weed out those that fall below 40.6 and allow those that fall closer to 41 (which AGS and GIA consider a fantastic pavilion) and 41.2+ or whatever... then I would believe it was more accurate. Like the range should be 40.8-41.4 or whatever.

I think the HCA is definitely biased against GIA pavilion angles...
 
Christina, I'm not the original Poster, it's not my stone :) ... Its just something I noticed - when I entered what I know to be excellent cut stones into the HCA.

This is a funny story

"There was an article done by Jonathon, I mentioned in your last thread and I haven't located it yet, but he did a study with a stone that GIA considered an EX cut, however the stone failed all the test that it was given. So based on that info the stone should have been a dog. He then coupled the 'dog' with an AGS000 HA ideal cut diamond and presented the two to several of his customers and asked them which stone that found more visually appealing. I cant remember how many he asked but I do remember that all but one chose the 'dog' as the more beautiful of the two diamonds. So, while all of these tests are helpful in weeding out a long list of online inventory, they aren't infallible and in the end your eyes will tell you if the diamond is a beauty or not. I really hope that someone will come along and be able to answer all your questions to your satisfaction, unfortunately I just don't have enough experience to explain the hows and whys. ;)) But, really I wouldn't get too stressed out, like I said before, you are working with a reputable vendor and have a good return poliy, you will know when you see it if it's the one. :)"
 
2023|1342062674|3232674 said:
I just get worried when people discredit stones based on HCA, when I know for a fact, that a 57% table, a 34 crown and a 41.2 pavilion falls directly in the centre of what GIA considers excellent (perfectly well well well within the range, right in the centre) and is also supported by the link Yssie attached.

But when you use the HCA, this pops out at a 3.

Currently the HCA pavilion range, discredits GIA bulls eye excellent cuts, BUT pavilions that fall below 40.6 (which apparently ALL labs agree is outside the range) at 40.4 an HCA pops out a 0.6.

I would think that output for pavilion angles on the HCA should be higher weed out those that fall below 40.6 and allow those that fall closer to 41 (which AGS and GIA consider a fantastic pavilion) and 41.2+ or whatever... then I would believe it was more accurate. Like the range should be 40.8-41.4 or whatever.

I think the HCA is definitely biased against GIA pavilion angles...
Click to expand...

Yes, for good reason though. Steeper pavilions are more likely to show leakage. Like John Pollard said, the pavilion (mostly) is what drives the light performance.
 
Overly steep pavilions yes, but also too shallow pavilions also show leakage...

There was a link somewhere showing no light leakage for a 34/41 and a 34/41.2 with images. This particular person decided their sweet spot was 34/41 but couldnt find a stone that matched those parameters, so settled on 34.5/41.1 or something...HCA would discredit this stone in this case - I wish could find the link...
 
2023|1342059139|3232657 said:
"In the most popular metrics discussed here: AGSL, GIA, HCA and AGA - average pavilion angles from 40.6 - 41.0 have the greatest approval cross-system. There are exceptions but that range is highly considered the safe-zone according to those metrics, as well as the consensus of the most high-achieving diamond cutters and research gemologists I know. For the record, the AGSL shows most 0 light performance candidates to be in that range. The HCA and AGA prefer the shallow side of that range (and a bit lower). GIA prefers the steeper side of that range (and a bit higher). This is why the GIA gives EX to steep diamonds the HCA rejects and the HCA gives thumbs-up to shallow diamonds the GIA system doesn't reward."

EXACTLY my point.

If you put into the HCA a pavilion score lower than this range you have just quoted as ideal 40.6 - 41, just using 34 crown and 40.4 (0.2 off) for the pavilion the HCA still gives it a score of EX EX EX EX and 0.6 on the output.

That is outside what all experts consider optimum yet HCA considers it amazing.
Click to expand...

I'd just mention that Garry Holloway is also an expert. His system is based on his own opinions and studies. While they may not agree completely with other systems that doesn't invalidate everything about it.

If GIA believes 41.2 is optimum and is its 'target' figure it seems unfair to discredit GIA and its millions of diamonds that it cuts, analyzes and looks at, yet allow a 40.4 that falls 0.2 below what AGSL, GIA, and AGA average as ideal as an excellent score of 0.6 - which is below what all labs to be 'subpar'.

HCA is on online tool, and seems to favour shallower pavilions over a lab GIA that has years of diamonds cutting and expertise.
Click to expand...

I think a good number of regulars here understand what you're saying. I believe I do, and I have no argument with your position. You acknowledge GIA as the world's foremost authority and believe their system is most sound. That's cool. But I live and breathe diamonds every day. I see them constantly, with cut as my specialty and primary focus. As a professional I have the right to choose my own set of standards for what I perceive to be the "Ideal" (or the "EX"). Put bluntly, after exhaustive scrutiny, 41.2 is NOT my "target" for PA. Neither is 40.5. For years I have selected diamonds for myself and my loved ones and the round brilliants in my wheelhouse of "YES!" fall within a more limited envelope than either of the metrics discussed here. That does not make me wrong, nor does not invalidate the GIA or the HCA. My paradigms happen to be different (read more narrow) than systems which were designed to cast a wider net. In the same spirit, the HCA metric happens to be different (read centered differently) than GIA's.
 
But 2023, we virtually never recommend that someone choose a GIA stone by numbers only. We always recommend getting an idealscope image which will show whether the stone has leakage.
 
From the HCA people themselves:

"Our studies into the GIA Cut Grading system and communications with the Cut Study Team have forced us to take a hard look at diamonds we once would not even consider. We currently haven’t made any hard core decisions about purchasing for inventory or not but we have had to take a step out of our shoes so to speak to see if GIA was out of their minds or not, suggesting what we have always considered certain steep/deep combinations as being “bright” and “fiery” stones. Admittedly I chuckled at the notion and was extremely skeptical until I received our first GIA Ex steep/deep which I was able to use in an observational survey we conducted next to a GIA VG (current AGS ideal) with painted girdle facets. Out of 30 observers we showed the comparison to 26.5 (we allowed them to pick a preference for brightness and fire) picked the GIA Ex steep/deep!!! That study caused me to rethink the whole steep/deep issue. Sergey of MSU came out and told me in a recent posting, THAT IS A FINE DIAMOND!?!? Where has Sergey been all this time on this issue? These truths I know have also penetrated the mind of Garry who is now also rethinking the HCA system. Both Garry and I (both research gemologists) are now realizing that it is wrong to lump all the stones we were considering steep/deep into one category. Particularly the 2 stones I had used in the demonstration earlier, shown again below."
 
John,

Thank you for taking the time to understand and read my post.

"I think a good number of regulars here understand what you're saying. I believe I do, and I have no argument with your position. You acknowledge GIA as the world's foremost authority and believe their system is most sound."

I very much respect that you work with diamonds every day, and your explanations that parameters from GIA, AGS, Garry Holloway, are just that, parameters, and that your own parameters are also different again. I understand some will vary and overlap and some will differ.

My husband told me that diamonds are non fungibles so they are not traded on the stock market (like gold) because of all the differences and nuances between them. This seems to solidfy that even more.

At the end of the day diamonds are a beautiful symbol of love :)

Again, thank you for listening to what I was saying and taking the time to respond.
 
2023 - have you actually seen what a stone with a PA of 41.2 deg looks like in person? :wavey: I have (a few actually). I returned it. I've read that article you posted before. Like 6 times. It made me feel OK about the stone I got with a 41.2 PA because it was close to Morse's parameters (I read a little on him). But in real life, I could see the effects of a 41.2 pavilion angle on the stone I had. It had that darn ring of death!

The author of that article concluded these parameters were best for a stone:

Listed in order of parameter importance:
1. Pavilion main angle = 41°
2. Length of pavilion halves = 77%
3. Crown main angle = 34°
4. Table size = 56%
5. Star Length = 55%
6. Girdle size = thin to medium
7. Culet size = small to none

I think many would agree the PA is the single most crucial number in minimizing leakage, and thus a has a great affect on light performance. GIA may state a 41.2 PA as being a sweet spot, but a handful of numbers that I plug into the HCA with that PA return slightly higher scores. I, personally, wouldn't look at stones above 41 PA, but that's never to say there aren't awesome looking stones out there with a 41.2 PA!


Mr. John Pollard - what say you about a 33 deg CA paired with a 15% crown ht and 40.8 deg PA? Is that considered shallow or borderline shallow?
 
Yes of course I’ve seen one in person, one that falls right into the exact target happy sweet spot for GIA excellent cut, 57% table, 41.2 pav, 34 crown, ex sym, ex polish...a blingtastic diamond, the type that shines from across the room which strangers compliment on and where arrows are clearly seen...absolutely no fish eye or dead circle. I also happen to think Hearts on Fire Diamonds are blingtasticly amazing too and yes, I’ve seen H&As in person. I also have blue boxes from Tiffany (not diamonds) – but my point is...I like Tiffany too. I like things to be validated - the GIA sweet spot is validated. I also like Bora Bora, specifically the overwater bungalows that look out at the mountain. That’s the sweet spot in Bora Bora where the St Regis and Le Meredian is. The majority of recommendations say that and my experience seconds that. If you went to Bora Bora and stayed on the main island, you probably wouldn’t like it, it’s still Bora Bora but it’s not the sweet spot. There may be another place on the island you find that is good too, if you stay in every part of the island you might find a better place, it may take me some convincing to believe it was going to be better than the current sweet spot, but you'd need some validation...which is why I completely understand the answer with regards to John saying he works with diamonds every day etc, and that AGS, GIA and HCA all have different parameters.

However, I don't know why the diamonds you bought made the fish eye, maybe the Pav paired with the cut, depth, table and crown made it worse... or maybe the 150 years of GIA cutting experience finding that sweet spot of 57% table, 41.2 pav and 34 crown, it’s a fluke only the lucky people get the good stones even when they are cut in that ideal sweet spot (for GIA) an incredibly reputable company.
 
" GIA may state a 41.2 PA as being a sweet spot, but a handful of numbers that I plug into the HCA with that PA return slightly higher scores."
Exactly, thats been my point this ENTIRE post, the HCA discredits the absolute sweet spot for GIA. The guys who made the HCA calculator even admit they were doing this and even thought they should reconsider the HCA tool.
People can get a 34 crown and a 40.4 pav and get a 0.6 HCA which is a stone that would be considered shallow by most.
Thats all :)
 
^the HCA favors slightly shallower stones, but Mr. Holloway has expressed the notion of possibly updating it.

But steep deeps should be avoided regardless. =)
 
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