Help! GIA Excellent Cut Vs. HCA

[yssie]

I've found in my time here that there's been a clear progression in what I think is the best way to choose a stone that hasn't/can't be reviewed by a trusted professional. I went from "trust your eyes" to "it's all about the tools and numbers" to "well, numbers don't tell you everything, but photos are generally good enough" to "I'm withholding all judgment until I see the thing in-person". Meaning that I'm right back where I started - I'll trust my eyes. Now my eyes have a much better idea of what to look for, so I'm confident that trusting them won't lead me astray.


2023/Titan - I agree that HCA can be too harsh on steeper pav stones - hence my personal rec for cutoff at 3 or 3.5 for "worth getting more info on", which is all that HCA is designed to tell you. I also agree that it is useless to quibble over a 0.1 or 0.2deg difference as presented on the report, after measurements have been averaged and rounded, knowing that the scanners the labs use are only accurate to 0.1deg (and from the sarin website I have absolutely no idea if that's w/ a 95% confidence or 99 or...)

The followup blurb is interesting. There are definitely different sorts of "overly deep" - DiaGem I believe stated that slight leakage under the table can yield more fire in various types of lights, and I've seen BS printouts on GOG that confirm that (yes, I know, BS is a whole different topic). It is a logical conclusion - white light is always going to be higher energy, it'll drown out proximal dispersions. Then we get into stone size, and from there into facet size - larger facets spaced further apart angled further away from each other are going to permit more visible coloured light return...

I think this article from Karl is really helpful w/ putting things in perspective. The LGF perspective specifically HCA doesn't consider LGF at all, and it can make a big difference in how a stone looks https://www.pricescope.com/journal/do_pavilion_mains_drive_light_return_modern_round_brilliant

I love seeing colour, so I've got no problem with some leakage under the table, if it's because of shorter lower halves. That isn't want most people who come to PS are looking for though. Since I love colour I'm also much more forgiving of deep high-crown stones than I am of low-crown/deep pav stones - so 36/41 would go into my personal "get more info on" bucket. But again, that isn't what most people who come to PS are looking for. I don't like painted stones, the folks at EightStar make a living off the practice, and most newbies don't care either way.


Hmm. Not sure what the point of that post was... well, maybe it'll give someone somewhere something to chew on.

 

[John P]

John,

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

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.

You're very welcome. Thanks for the follow-up.

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.

You're welcome again. And thank you for thanking me.

RE the stock market, your husband's intuition is correct. It's a bit off-topic, but there are three primary reasons diamonds can't be treated like individual stocks: First, the public doesn't have access to diamonds at B2B prices. Second, as he deduced, a half-dozen rounds all categorized as "1ct G SI1 EX" can vary in both value and appearance. And third, the current differences in lab grading-standards, retail sales-information and pricing systems creates opacity that's hard to penetrate, making valuation extremely difficult for consumers (and even some pros I encounter).

It may be of interest to note that diamond volatility has proven quite low, historically, while prices have generally increased. As a result, and certainly a sign of the times, there are practical investment vehicles appearing. Rather than fighting downstream opacity, the practical programs provide an opt-in at a far upstream level. Some are index-funds designed to return a % based on price-increases, other programs provide other options.

 

[John P]

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."

As a point of order, this wasn't written by anyone associated with the HCA. I believe it was composed by Jonathan Weingarten as a rebuttal to a certain range of HCA results - in much the same spirit as the views you've expressed.

 

[John P]

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!

There are. It's about so many things, however. The crown-pavilion pairing is key. Details of the table and lower halves are critical. In some cases an adjustment to the brillianteering of the upper halves can enhance the performance and the 3D precision of the diamond can absolutely influence the resultant optics.

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?

Cool combination. For me it's not shallow; the 15% crown height takes it out of shallow territory. But to have that 15% crown height with a 33 CA the table would have to be cut only 53.5% wide. With precise cutting and proper lower-half length for wide pavilion mains such diamonds can have a beautiful transitional/antique look while remaining bright.

As a FYI, neither GIA or AGSL would give a 54/40.8/33 configuration their top cut grade - if it matters.

 

[Christina...]

2023 I apologize for getting you confused with the OP and therefore much of what I said was irrelevant to you specifically. But I'm still confused as to why you think that a 57/34/41.2 is the 'sweet spot' of all of the stones that GIA considers for the EX cut class. Simply because these proportions fall within the middle of the graph doesn't mean that these proportions will perform any better than any of the other proportions that also received an excellent cut grade. Dead center of Bora Bora may be a gas station restroom if you know what I mean.

That said, HCA remains the most efficient tool available to weed through the enormous online inventory and the best way for consumers to establish a short list. We always explain to new consumers that it is NOT a selection tool and should be used only to determine a stones predicted light performance. And that there are other tools that will better predict the stones overall potential, but the list has to be narrowed somehow, and I think that we are fortunate that such a tool as the HCA exists.

 

[TitanCi]

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!

There are. It's about so many things, however. The crown-pavilion pairing is key. Details of the table and lower halves are critical. In some cases an adjustment to the brillianteering of the upper halves can enhance the performance and the 3D precision of the diamond can absolutely influence the resultant optics.

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?

Cool combination. For me it's not shallow; the 15% crown height takes it out of shallow territory. But to have that 15% crown height with a 33 CA the table would have to be cut only 53.5% wide. With precise cutting and proper lower-half length for wide pavilion mains such diamonds can have a beautiful transitional/antique look while remaining bright.

As a FYI, neither GIA or AGSL would give a 54/40.8/33 configuration their top cut grade - if it matters.

You're a pure genius. The table is 54%, the depth is 61.7 (or 61.8 I can't remember) but on paper it gets a GIA XXX. What gives? Please explain! Stars 50, LH 80% if that matters...

 

[2023]

Hi Christina, because a lot of people on this forum discuss the broader ranges of GIA cutting parameters vs AGS as an explanation as to why an excellent GIA stone does not pass HCA. Quite candidly in fact. The graphs I looked at showed a sweet spot for GIA and was also the spot most closely aligned to AGS centre ideal, they also named the sweet spot, so my point was a stone that meets GIA xxx and AGS ideal should pass HCA. The authors also referred to it as the sweet spot themselves, 41.2/34 for GIA and 41/34 for AGS (Im not the author) Obviously there is a whole bunch of other stuff going on to make a diamond beautiful.

Also the OP, his stone sits right on the edge of GIA ex, and so I thought many people may say 'that's why it didn't pass HCA' also, the stone he has (with only pav and crown angles) on numbers only comes up as an AGS good...so I thought people may again use that reasoning why HCA didn't work for him. Hence why I used an example closest to both AGS ideal and GIA ex ex ex.

I'm no mathematician but I understand most cutters have a parameters of dimensions to cut with and to not deviate too far from these dimensions. To avoid the fish eye if too shallow, or a dark diamond if too deep. E.g if you use tolks hypothesis of the most perfectly cut diamond, people would try to aim to get as close to that as possible.

A scatter graph gives the relationship between two variables and how they relate to each other, if you asked someone when is the best time of year to go to bora bora versus what you should wear, a linear relationship would show possibly when it's winter wear jeans, when it's summer wear a bikini, when its spring wear shorts...the scatter plot simply gives the trend curve. A normal distribution curve gives the mean. Dead centre of a normal distribution curve is different to dead centre of a geographical area of bora bora. If you asked 100 people would you rather be going in winter and wearing jeans, spring and shorts, or summer and bikini, the mean of this answer is what the majority of people answer and it may be shorts and spring. So then you could deduce on the scatter graph the sweet spot is spring and shorts time.

I never gave the sweet spot the authors did, and I assumed they did it based on cutting parameters, averages of diamonds that fell within those parameters that were classified as excellent or ideal cuts, I'd need to read the references to see how they got their exactly.

HCA has their sweet spot, John has his, the paper indicated the sweet spot for AGS and GIA (not me!). My only reason for even discussing sweet spots was to dispute the HCA. Even tolks diamond dimensions are just an hypothesis.

Anyhow, getting all over the place now, as ysie said very insightfully she has moved from numbers, to sight to professionals, to her own eyes etc.

Which makes me think, I hope to reply to John and get Fife hopefully an answer as I feel I've hijacked his post.

 

[2023]

Hi John,

I knew that we should be investing in diamonds

If you are still reading this thread - as I feel as though I've hijacked it. The original poster has bought a 1.5 carat VS1 H colored diamond GIA 41/36 57 table 62.4 depth, sight unseen so far, which he has paid for but can return. If you could offer him some advice I'm sure he would appreciate it.

Thanks a million for your time and professional input.

PS fife congratulations on your upcoming engagement!

 

[2023]

2141708547 (GIA cert)

The Original Poster did post a link to his GIA in his first post on this thread and there are images too. The diamond looks like its on a slight tilt, and the ideal scope image looks a bit dark...maybe the jewellers head was obstructing some of the light? But there may be some information on there and comments for fife to learn more about the diamond.

Kind Regards
2023

 

[Md123456789]

My stone has a 'bad' HCA score and is a GIA excellent cut. I love it and literally get comments on it daily. I think that being able to see it in person gave me the confidence to love it. Trust your eyes!

 

[TitanCi]

Cool combination. For me it's not shallow; the 15% crown height takes it out of shallow territory. But to have that 15% crown height with a 33 CA the table would have to be cut only 53.5% wide. With precise cutting and proper lower-half length for wide pavilion mains such diamonds can have a beautiful transitional/antique look while remaining bright.

As a FYI, neither GIA or AGSL would give a 54/40.8/33 configuration their top cut grade - if it matters.

John - why does the stone get an excellent cut grade on the GIA report then? Not just FW...

 

[2023]

Hi Christina, more on the 'sweet spot' for you
http://www.acagemlab.com/articles/SweetSpot.htm

 

[Christina...]

Hi Christina, more on the 'sweet spot' for you
http://www.acagemlab.com/articles/SweetSpot.htm

This 'new' article doesn't have any new information, I read the original when you posted it the first time. My point was that purchasing a diamond based on the proportions landing dead center of a range of what is considered excellent is a really bad idea. Simply because the chart shows that the middle or 'sweet spot' as the author likes to refer to it, is a 56/34/41 combo, does not mean that diamonds cut to these proportions have better optics or are more beautiful than a stone cut with any of the other proportions listed with the ideal range.

As a member of the trade. John isn't allowed to comment on a specific stone.

 

[Christina...]

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."

I also just wanted to point out that this article was written many years ago, I can't remember specifically 2005 maybe? I obviously can't speak for Jonathon or Garry, but if Garry was indeed rethinking the HCA system and how it grades steep PAs, he apparently decided not to do so. You should start a thread asking him why specifically he chose not to. He's very good about discussing the uses and limitations of his tools.

edit: Just one more thought, there are many occasions when an AGS0 has scored above a 2 on the HCA as well, so it's not only happening with GIA ex stones.

 

[TitanCi]

^But all I'm asking about is specs.

^Also, you're right Christina, I've seen some fugly AGS000 "Ideals", and I'm talking about old ones; I'm talking about newer stones.

 

[Christina...]

^But all I'm asking about is specs.

^Also, you're right Christina, I've seen some fugly AGS000 "Ideals", and I'm talking about old ones; I'm talking about newer stones.

I didn't mean YOU Titan. 2023 was asking John to give his opinion on the OPs stone. I was just pointing out that he couldn't post about that one in particular. Your's is still a hypothetical stone.

I actually haven't seen an ugly AGS0, but I haven't seen many IRL either, but I have seen them score very low on the HCA on many many occasions. I was just trying to show 2023 that the HCA wasn't biased specifically to GIA EX stones and that we see the same thing happen when scoring AGS0 stones as well.

edit: I meant high on the HCA above 2, not low.

 

[John P]

The table is 54%, the depth is 61.7 (or 61.8 I can't remember) but on paper it gets a GIA XXX. What gives? Please explain! Stars 50, LH 80% if that matters...

Interesting Titan. GIA originally gave 54/40.8/33 VG, regardless of other factors. Clearly since 2006 they've widened this tolerance (graphic from my GIA DG textbook).



Upon further investigation I see they have made the same adjustment for table sizes 52-57, which were also VG in prior versions.

 

[John P]

RE Titan's 40.8/33 example at Tolk table sizes:

It's a compelling case to me. Expanding the envelope a bit, lengthening the stars to 55% causes the diamond to fall into the VG range. With this crown configuration it's certain the stars can play a greater role than they might in others.

Check these ASET simulations at 54/40.8/33. The minors in the top graphic are at 50/80 (disregard the DiamCalc GIA-V, since Titan demonstrated this combo has been revised to receive EX). In the bottom they're at 55/80.

I think it's pretty interesting. Any observations?

 

[Christina...]

RE Titan's 40.8/33 example at Tolk table sizes:

It's a compelling case to me. Expanding the envelope a bit, lengthening the stars to 55% causes the diamond to fall into the VG range. With this crown configuration it's certain the stars can play a greater role than they might in others.

Check these ASET simulations at 54/40.8/33. The minors in the top graphic are at 50/80 (disregard the DiamCalc GIA-V, since Titan demonstrated this combo has been revised to receive EX). In the bottom they're at 55/80.

I think it's pretty interesting. Any observations?

Ok, I'll take a stab at it.

It appears that the stone will be much brighter under the table with the 50/80 combo and less bright with the 55/80 combo?

 

[John P]

Ok, I'll take a stab at it.

It appears that the stone will be much brighter under the table with the 50/80 combo and less bright with the 55/80 combo?

Ding ding ding ding ding... Do people still use that to mean "correct!"? - Forgive the old TV game-show culture.

It's certainly that way. While some might argue that the difference is extremely slight these simulations show that it does exist. There is still another difference which appears to occur; one that might seem to be contrary to your conclusion...? That's what is interesting to me.

 

[Christina...]

Ok, I'll take a stab at it.

It appears that the stone will be much brighter under the table with the 50/80 combo and less bright with the 55/80 combo?

Ding ding ding ding ding... Do people still use that to mean "correct!"? - Forgive the old TV game-show culture.

It's certainly that way. While some might argue that the difference is extremely slight these simulations show that it does exist. There is still another difference which appears to occur; one that might seem to be contrary to your conclusion...? That's what is interesting to me.

I'm not sure, but I feel like I'm seeing much more contrast in the 55/80 combo and I know that some contrast can make the rest of the stone appear brighter, but I would have thought the contrast showing in the stars would result in a darker appearance in the stone, but it does appear to have larger areas of brightness between them?? Am I'm missing something obvious?

 

[John P]

Hi John,

I knew that we should be investing in diamonds

If you are still reading this thread - as I feel as though I've hijacked it. The original poster has bought a 1.5 carat VS1 H colored diamond GIA 41/36 57 table 62.4 depth, sight unseen so far, which he has paid for but can return. If you could offer him some advice I'm sure he would appreciate it.

Thanks a million for your time and professional input.

You're very welcome 2023.

As a trade professional Pricescope forum policies request that I refrain from endorsing or criticizing a specific diamond when the vendor is known; in order to remain neutral as it relates to different sellers. But there are plenty of consumer enthusiasts who can comment away.

 

[John P]

I'm not sure, but I feel like I'm seeing much more contrast in the 55/80 combo and I know that some contrast can make the rest of the stone appear brighter, but I would have thought the contrast showing in the stars would result in a darker appearance in the stone, but it does appear to have larger areas of brightness between them?? Am I'm missing something obvious?

You're not missing anything. It's as you noticed; the 55/80 configuration has less-concentrated brightness under the table, but greater brightness/contrast in the overall profile and what I perceive as a reduced lower-angle draw. The reason it's interesting to me is the potential to represent a situation where GIA bumped-up the top example but if AGSL was asked to bump one or the other they might go the other way. Based on the static ASET only, it looks like it could have a higher overall raw-data-score. Maybe, maybe not. I suppose I could ask the lab to run these sims.

And before I mislead anyone as to the importance of this; it's not important at all!

It's nitpicking in the extreme, especially since ASET is a very simple component, measuring brightness and contrast only and seen in one static position here. So no hard conclusions should be drawn, but I think it's cool to juxtapose these examples. First, since GIA chose to move this particular threshold and second, because I'm totally a nerd.

 

[Laila619]

Huh, I didn't realize stars could have that much of an effect. I thought they were pretty minor.

 

[Christina...]

I'm not sure, but I feel like I'm seeing much more contrast in the 55/80 combo and I know that some contrast can make the rest of the stone appear brighter, but I would have thought the contrast showing in the stars would result in a darker appearance in the stone, but it does appear to have larger areas of brightness between them?? Am I'm missing something obvious?

You're not missing anything. It's as you noticed; the 55/80 configuration has less-concentrated brightness under the table, but greater brightness/contrast in the overall profile and what I perceive as a reduced lower-angle draw. The reason it's interesting to me is the potential to represent a situation where GIA bumped-up the top example but if AGSL was asked to bump one or the other they might go the other way. Based on the static ASET only, it looks like it could have a higher overall raw-data-score. Maybe, maybe not. I suppose I could ask the lab to run these sims.

And before I mislead anyone as to the importance of this; it's not important at all!

It's nitpicking in the extreme, especially since ASET is a very simple component, measuring brightness and contrast only and seen in one static position here. So no hard conclusions should be drawn, but I think it's cool to juxtapose these examples. First, since GIA chose to move this particular threshold and second, because I'm totally a nerd.

So, IRL the 55/80 would have the same potential for brightness as the 50/80 in your example? Like, Laila I didn't realize that the stars had that much visual impact on the performance of a stone...that's really interesting.

 

[John P]

So, IRL the 55/80 would have the same potential for brightness as the 50/80 in your example? Like, Laila I didn't realize that the stars had that much visual impact on the performance of a stone...that's really interesting.

All else equal, with a 5% star length the only difference, I doubt any human being could discern differences in general performance. But they have to put the threshold somewhere.

RE stars & visual impact: Round brilliants along the main cutter's line have overall performance dictated more by their three primary components (T, PA, CA). As diamonds stray from that line their optics can be more influenced by variations in the minors.

 

[TitanCi]

Mr. Pollard - many thanks for the replies and insight into the particular stone. The graphics and your analysis are interesting, and perhaps my example helps show why the HCA may need to be revised (although it scores a 0.80 on the HCA). And also the fact there is discussion about an AGS Ideal that doesn't too well on the HCA either!

Perhaps many still need to grasp the fact GIA's excellent range is too broad, but it opens up for different "makes" and tastes in stone appearance. Thus, the HCA helps to weed out potentially ugly stones.

2023 - I don't doubt GIA's sweetspot is tried and true, but the fact the range is so broad is what sucks for most consumers

What a great topic that I'm sure has been beaten to death with many threads.

 

[2023]

Christina, of course Im not saying one stone is better than another based on exact numbers, THATS been most of my point of this entire thread 'pidgeonholong' of the HCA and using examples that fell within the parameters of AGS0 and GIA ex to dispute that so no one would say GIA is broader on cutting etc. HCA also uses a range and has their sweet spot, and are doing exactly the same thing as these authors and their sweet spots, BUT plugging in the numbers on an online tool AND people on this forum over analyse stones by 0
2 - 0.5 of a percent or degree. GIA sees the stones, look at them under light, measure them and then give a grade of ex or whatever, which is one if the reasons they are seen to have broader cutting metrics.
My only point was a stone AGS0 and GIA ex on that zenith point can fail HCA which seems a bit odd.

By the was I thought my explanation of a scatter plot was great!!!

Fife may need to start another thread with his ideal scope image (though his diamond does look like its on a slight tilt and the image is a bit dark like the jeweller had his head in the way).

 

[2023]

Christina, as John says here

"Round brilliants along the main cutter's line have overall performance dictated more by their three primary components (T, PA, CA). As diamonds stray from that line their optics can be more influenced by variations in the minors."
...would mean there are parameters.

I'm not saying stones that fall right in the centre of a certain range are the best and most amazing, it's just a guidance that cutters I believe try not to stray to far from. Like in the scatter plot example I used, some people may love Bora Bora in summer wearing shorts, even if the other factors indicate spring and shorts. Also not one person or group or lab has been able to decide on the ideal diamond. it's got math but it's also subjective, as John said his parameters differ again.

Like who is the most handsome celebrity, or lets take an industry that is based on parameters but also subjective, like the modelling industry. Take two successful agencies with a group of models, one only chooses those over 5ft 8 and below 5ft 10, weight 50-55kg the other might be broader and choose 5ft 7 to 5ft 11, weight 47-57kg and some may overlap and work for both (those that are 5ft 10 and 55kg). If there was an online agency (the HCA) who automatically cut off at the 5 ft 10 group and 55 kg group yet went down to 5ft 7 models and 58 kg (outside what both agencies selected), I might question that, especially if they hadn't even seen the models! Also the most successful as you say may or may not be in the five ft 10 group 55 kg group but someone in the five ft 8 group might have the most successful career, they are still models and are in that agency as someone who has seen them has decided they should be a model and this five ft 8 model might have amazing hair or something! Although many designers may prefer working with the 5 ft 10, Kate Moss is exceedingly famous and is just 5 ft 7.

As we know so many factors go into a diamond, in this thread we were discussing two points, crown and pav angles where there are simply just parameters.

Titanci - that's great you think the HCA could be relooked at (I was worried everyone would just say HCA HCA HCA!!!!)

Also yssie and her 3.5 seems logical and I also read once dreamer looks at over 1-2. So 1-3.5 would be their range.

 

[Christina...]

Christina, as John says here

"Round brilliants along the main cutter's line have overall performance dictated more by their three primary components (T, PA, CA). As diamonds stray from that line their optics can be more influenced by variations in the minors."
...would mean there are parameters.

I'm not saying stones that fall right in the centre of a certain range are the best and most amazing, it's just a guidance. Like in the scatter plot example I used, some people may love Bora Bora in summer wearing shorts, even if the other factors indicate spring and shorts. Also not one person or group or lab has been able to decide on the ideal diamond. it's got math but it's also subjective, as John said his parameters differ again.

Like who is the most handsome celebrity, or lets take an industry that is based on parameters but also subjective, like the modelling industry. Take two successful agencies with a group of models, one only chooses those over 5ft 8 and below 5ft 10, weight 50-55kg the other might be broader and choose 5ft 7 to 5ft 11, weight 47-57kg and some may overlap and work for both (those that are 5ft 10 and 55kg). if there was an online agency who automatically cut off at the 5 ft 10 group and 55 kg group yet went down to 5ft 7 models and 58 kg (outside what both agencies selected), I might question that, especially if they hadn't even seen the models! Also the most successful as you say may or may not be in the five ft 10 group 55 kg group but someone in the five ft 8 group might have the most successful career, they are still models and are in that agency as someone who has seen them has decided they should be a model and this five ft 8 model might have amazing hair or something!

As we know so many factors go into a diamond, in this thread we were discussing two points, crown and pav angles where there are simply just parameters.

Titanci - that's great you think the HCA could be relooked at (I was worried everyone would just say HCA HCA HCA!!!!)

Also yssie and her 3.5 seems logical and I also read once dreamer looks at over 1-2. So 1-3.5 would be their range.

Well, it's been a very long run around trying to come to some sort of agreement. I've never maintained that the HCA is the be all end all, simply that when navigating through an enormous list of online inventory it remains the most effective tool we have to eliminate predicted non-performers and aids in creating a short list. And certainly some beautiful stones fall through the cracks and some ugly ones pass. No one has to my knowledge ever suggested that a person purchase a diamond based on the fact that it scored under two on the HCA, only that they proceed to gather more information, ie an idealscope image which is much more accurate at determining the overall performance of a stone. I don't agree that simply moving the line though fixes the problem. If the line were to be moved to 3.5 then we could be having this identical conversation with someone who's stone falls at 4 or 4.5 kwim? If no line is established then the HCA no longer works and any stone with a GIA EX or AGS0 warrants further investigation and I think that we agree that isn't and shouldn't be the case. AGS performs their own light performance testing on all of their AGS0 stones, eliminating the need to run them through the HCA, perhaps GIA should consider doing the same? I'm happy that John was able to chime in, I learn something from him every single time that he posts. He's a fantastic contributor. I was hoping that Garry himself would pop in and join the discussion