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HCA, AGS and GIA

2

2023

Shiny_Rock
Joined
May 23, 2012
Messages
201
Hello,

I've been a bit skeptical of the HCA (weeding out some great diamonds and letting others through).  This may be a really silly question with an obvious answer but I've just been playing around with some angles and I'm still confused with the system  :confused:  I understand its a blunt tool to weed out diamonds, but can't help but think that some diamonds get further investigation that shouldn't and others that could be great diamonds don't. 

Okay, so I put in a range of pavilion and crown angles (some silly I know)
39.8 pavilion 39 crown HCA score 1.4
40 pavilion 39 crown HCA score 1.3
40.2 pavilion 37 crown HCA score 1.3 
39.6 pavilion 35 crown HCA 1.7

41/34 gets an HCA of 1.3 then..
41.2 pavilion 33.5 crown HCA score 2.3 (would be disregarded)
41.2 pavilion 34 crown HCA score 2.8
41 pavilion 35 crown HCA score 2.1 
41.4 pavilion 33 crown HCA score 3

What confuses me the most is that all the stones that scored above 2 (in the 41ish group just based on pavilion and crown angles are candidates for AGS0 ideal diamonds and GIAex) on the other hand those that fall below 2 are either poor, fair or VG diamonds (just based on the angles for AGS and GIA).

I understand AGS updated their parameters after the HCA was created, and I've read in threads the Gary Holloway had thought that the new cut guidelines would be more in line with the HCA, but from these examples that doesn't seem to be the case.

I'm hoping someone will be able to shed some light on this.
 
The HCA also needs the table size and the depth of the stone so you can't just input crown and pavilion angles. The HCA dings most stones with a pavilion angle of greater than 41.

What table and depth are you using for these HCA scores? You would not usually see stones cut within the parameters of your first group - they wouldn't be very pretty and they wouldn't get AGS or GIA top cut grades.
 
I used a table size of 57%

Using the AGS and GIA tables from the octonus website guides of pavilion and crown angles and table percent.

http://www.octonus.ru/oct/mss/gia-agspgs.phtml

40/39, table 57%, depth 65% HCA 1.7
40/39 table, 57% depth 60% HCA 1.2
40/39 table, 57%, depth 63% HCA 1.6

41.2/33.5 table 57% depth 60% HCA 2.3
41.2/33.5 table 57% depth 62% HCA 2.6

The 41.2/33.5 combo would be considered ideal by AGS and Ex by GIA but would be disregarded on the HCA.
 
What is the confusion?

There is an inverse relationship between crown and pavilion angle that needs to be maintained. The AGS 0 candidate combos are on the HCA chart, and you will see that the same inverse relationship exists on the AGS system.
 
I'm not going to analyze all the JA stones, but for example:

http://www.jamesallen.com/diamonds/K-VS1-Very-Good-Cut-Round-Diamond-1416784.asp
57/37/40.2 GIA

This one has a depth of 63.5% and we'd never recommend the stone because it faces up like a .80 ct or so. I wouldn't ever have entered it into the HCA because the stone wouldn't be acceptable for obvious reasons.

Too deep usually equals a stone that faces up smaller than it's weight. I try to buy stones that are 60-62% in depth but no deeper than 62.3.
 
The confusion is that diamonds that can be considered both AGS ideal and GIA ex simultaneously can be rejected on their HCA scores.

Exactly, using the inverse relationship theory tolkowsky had a 40.75 pav with a 34.5 crown, so a 41 pav and a 34 crown would be inversely proportional, so a 41.2 pav and a 33.5 would also be inversely proportional but would be rejected on the HCA scoring a 2.5 (was in my pp)

41.2/33.5 is an AGS ideal and GIA ex candidate simultaneously but is rejected by the HCA.

The other diamonds which I listed, that would score under 2 on the HCA are considered poor cuts/fair cuts by AGS and GIA, yet are being sold as 'good diamonds' which I think was why yssie listed the diamonds,' yet a 41.2/33.5 would be disregarded.

My understanding, and correct me if I am wrong is that AGS and GIA both use computer modelling to identify their excellent and ideal diamonds, AGS I believe is a 3D computational model. AGS and GIA also look at the diamond under lighting conditions in person.

There are numerous examples that people have posted that are AGS0 that fail the HCA.

Not meant to 'flame' at all, am just really confused why these stones would fail the HCA. Is it possible that it's even a way to sell less desirable candidates (as so many people rely on the HCA here or is it to get a bargain) because other more high end stores would not stock those diamonds because they don't meet ideal and ex standards and are less desirable?
 
2023|1343360773|3241141 said:
The confusion is that diamonds that can be considered both AGS ideal and GIA ex simultaneously can be rejected on their HCA scores.

Exactly, using the inverse relationship theory tolkowsky had a 40.75 pav with a 34.5 crown, so a 41 pav and a 34 crown would be inversely proportional, so a 41.2 pav and a 33.5 would also be inversely proportional but would be rejected on the HCA scoring a 2.5 (was in my pp)

41.2/33.5 is an AGS ideal and GIA ex candidate simultaneously but is rejected by the HCA.

The other diamonds which I listed, that would score under 2 on the HCA are considered poor cuts/fair cuts by AGS and GIA, yet are being sold as 'good diamonds' which I think was why yssie listed the diamonds,' yet a 41.2/33.5 would be disregarded.

My understanding, and correct me if I am wrong is that AGS and GIA both use computer modelling to identify their excellent and ideal diamonds, AGS I believe is a 3D computational model. AGS and GIA also look at the diamond under lighting conditions in person.

There are numerous examples that people have posted that are AGS0 that fail the HCA.

Not meant to 'flame' at all, am just really confused why these stones would fail the HCA. Is it possible that it's even a way to sell less desirable candidates (as so many people rely on the HCA here or is it to give people a bargiain) because other more high end stores would not stock because they don't meet ideal and ex standards and are less desirable?
Click to expand...


No, nothing so specific. Just throwing them out for the wolves - or not, as the case may be.

There are lots of relationships all working together, you really can't try to hold everything else constant and just change one or two variables and expect useful trends - this is a very, very nonlinear series of equations, that's kinda why I posted the last two specifically - just adding a third variable (table) changes your result. Like DS says there's a lot more to choosing a stone than just the HCA inputs - girdle, LGF, age of report, inclusion visibility & effects, brillianteering...

Are you a math person? I can probably explain what I'm trying to say better functionally - in the above (holding everything constant and varying the one variable) you're basically taking the partial derivative of the equation wrt whatever you're varying - so if you've got say (d for the partial) df/dx x^2 + y = 2x then things are great because it doesn't matter what your y is, if you just vary x then whatever trends you get are true of the entire system.
But if you've got something messy like df/dx (x^2)(y^2) + y = 2xy^2 the trends as you vary x depend very much on what your y actually is - if y is 0 the whole thing zeroes out, if y is 1 it's linear, if y is 2 it's parabolic! And w/ diamond proportions the whole darn thing is interdependent.


ETA: my understanding is that GIA averages around eight sections of diamond, then rounds, then assigns a proportions-based cut grade based on those rounded averages. The AGSL charts are just basic guidelines - a stone can earn a 0 even if the proportions aren't listed in the 0 range on the chart depending on PGS ray tracing analysis
http://www.americangemsociety.org/performancegradingso
 
Have you seen this chart? https://www.pricescope.com/wiki/diamonds/holloway-cut-advisor HCA also does not taken into account stereo vision (where the image from each eye is slightly different, and then the brain puts them together into one seamless image). Taking that into account, stones that score up to a score of 3 are well worth looking into.

The AGS system is definitely for young people/rings, not the old people/pendants/earrings section.

GIA system is rubric-based.
 
Nope no math person, sorry, that sounds like gobbledygook to me. But I can read a chart and understand basic maths. I may be more skeptical than the average person because I have background in research and science but work in sales and marketing, in an industry which is very heavily leveraged in evidence based medicine. Seeing both sides, the research and studies and the marketing and sales...I do question things that seem weak.

My question is why diamonds that fit parameters of an AGS and GIA ideal and ex candidate are so swiftly disregarded, just reading a simple graph, off octonus, where the results are very obviously correlated that a child could see a trend in AGS and GIA ideal and ex yet these results are discarded by the HCA.

The HCA is a very simple tool with only 4 variables, the graphs are very simple with only 3 variables...they should relate to each other more. The GIA and AGS graphs relate.
 
2023|1343364598|3241186 said:
My question is why diamonds that fit parameters of an AGS and GIA ideal and ex candidate are so swiftly disregarded, just reading a simple graph, off octonus, where the results are very obviously correlated that a child could see a trend in AGS and GIA ideal and ex yet these results are discarded by the HCA.

The HCA is a very simple tool with only 4 variables, the graphs are very simple with only 3 variables...they should relate to each other more. The GIA and AGS graphs relate.
Click to expand...


... the three systems aren't supposed to mirror each other, they aren't designed to mirror each other, and that's perfectly okay!

I'm kinda getting the feeling that the bigger issue is really What Is PS For. In MY opinion the goal is to steer new buyers toward making "educated purchases", unless they explicitly state otherwise. And MY definition of an "educated purchase" is not to just find a stone that sits in the middle of the AGS/GIA/HCA/AGA/whatever venn diagram, it's to encourage buyers to figure out what THEY like and help them find that. Sometimes that means going for the middle, sometimes it doesn't. And in MY opinion HCA does not need to have more overlap with GIA and AGS to be a useful tool. Obviously not everyone shares "MY" opinion!

Why I feel that way:
1. Stones in the middle aren't prettier, better performers, more nuanced, livelier than stones not in the middle. There are lots of flavours of lovely stones.
2. If you limit yourself to that small scope you're excluding a LOT of nice stones. This can be insignificant if you're looking in a popular size/clarity/colour set, but if there aren't a ton of stones to choose from you've just eliminated a huge number of candidates.
3. You're limiting yourself to one or two specific flavours. Some people (me) really like butter pecan, better than vanilla/chocolate/strawberry, and some people (me) don't particularly care for chocolate.
4. Youre missing out on any deals that might come about b/c of slightly unorthodox proportions.
5. GIA, AGSL, and HCA don't consider optical symmetry, only facet meet symmetry. So if symmetry of facet patterning (H&A and whatnot) is important to you you're still going to have to do some scouting on your own.

That said, it's
1. An effective way to find a nice stone, if there are enough left to choose from after culling everything else (ignoring the optical symmetry issue)
2. An efficient way to find a nice stone, if there are enough left to choose from after culling everything else (ignoring the optical symmetry issue)
So if a certain buyer's priorities are to get a good one as quickly as possible, even if it costs a bit more, it's a useful strategy.

It's usually pretty clear in the OP whether a buyer just wants a good one this week or whether he/she is interested in delving into the details.


To the specific issue here - that HCA is hard on steeper pavs is a well-known and well-documented issue, but you seem to have deeper misgivings. What's your suggestion?
 
To not automatically disregard a diamond that is above 2 on the HCA, that is a GIA ex stone where pavilion and crown angles correlate with an AGS ideal, for example the pp 41.2/33.5 where the HCA score was 2.5 should at least be 'investigated further' and not disregarded.

I'm not sure how high, maybe up to a 3 (like what a pp said) I haven't looked at enough combinations but to at least include diamonds that AGS ideal and GIA excellent agree with regards to their 'inverse relationship' on crown and pav angle a fair go.

Why knock out potentially amazing stones that have been analyzed by a reputable lab and given those grades for a reason.

I'm not using the 'ven diagram' to say these are the best diamonds, but it gave a better case if both labs GIA and AGS agreed, so people wouldn't argue the broader GIA cut etc.. Also most cutters do try to cut to certain ideals and proportions or these ideals wouldn't exist.

The HCA gives diamonds a go that AGS and GIA would not rate very high, and no doubt they may be great diamonds! But it seems wrong to not investigate further what AGS and GIA would consider Ideal and Ex, where outside of PS high end jewellers may very well sell in their shops!!!
 
So you want to see the official cutoff increased? Right now HCA guarantees success as best it can (w/ minimal inputs) at the expense of missing out on nice stones. To increase the cutoff would change the type of tool - you're going to catch a lot more of those nice ones but you're also going to fail to exclude some uglies.

HCA was the first of its type - created in 2000 I think? In mid-2000s GIA was just getting on board with including a cut grade at all... a conservative, ultra-nitpicky HCA would've been invaluable. But I would agree that now with so many vendors able and willing to provide more info - and since most e-vendors do let you ship out and return if necessary without a lot of drama - there's no real reason not to let the "further investigation" bit catch the bogeys that HCA might miss.
 
2023,

Surely, all systems can be criticized and you are pointing out confusion with valid reason. Most probably, if you understand the basics, background and limitations of each metric, part of the confusion will be solved.

1. The HCA was designed by Garry Holloway and uses very simple averaged numbers. The tool as such is mainly an indication of potential brightness only, not of potential fire or scintillation, and most probably because of personal preference of Garry, is favorising on the end of more spready stones, and thus more shallow combinations.

2. The GIA-system is based upon mostly industry-observations, and uses averaged numbers, highly comparable to the HCA. In the same way, the metric is an indication of potential brightness only, not of potential fire or scintillation. Possibly because of industry-bias in the design, it favors more steep/deep-combinations.

3. The AGS-system is science-based, and uses the actual 3D-scan of the stone. However, with the scientific studies of AGSL still in process, the metric is mainly an indication of potential brightness only, not of potential fire or scintillation.

Live long,
 
Just a little observation for the record that many observers and commentators perhaps miss.
HCA is the only system that gives a positive score for spread. This is based on a simple observation that many consumers prefer a larger diameter.
So since my system adds up penalty's and gives a total score, where as the other mentioned systems do not, it is an obvious side effect that HCA will give a preference to stones with larger diameters for their weight and this means that there will be a slight preference directed to shallower stones. However per se my personal preferences are not that far from Tolkowsky - adjusted slightly shallower because it is what ladies want: i.e.

Size does count.
Bigger is better!

In the future when making comments about HCA I hope the experts and prosumers here will take this into account.

for further original 10 year old commentary http://www.diamond-cut.com.au/14_spread.htm

I might add that my system is the only one that also relates the gridle thickness risk of chipping to other individual proportion risks such as shallow crown angle. So just going shallower and spreadier with those two factors is more than compensated for by chipping risk penalty.
Regarding this comment on that page
We choose to give a zero penalty for a diamond with a 40° pavilion, 32.5° crown, 58% table and a 1% girdle, diamonds with better spreads also rate zero. The worst score of 2.0 is given to a diamond with a 10% girdle, 40° crown, 54% table and 43° pavilion. Stones with even worse spreads are not further penalized.
I have since adjusted the spread factor, but since this site is an original paper - I do not like to edit it. I do not have the current mimimum with me on vacation, but it is more like 40.5 and 32.5.
 
Garry H (Cut Nut)|1343483436|3241977 said:
Just a little observation for the record that many observers and commentators perhaps miss.
HCA is the only system that gives a positive score for spread. This is based on a simple observation that many consumers prefer a larger diameter.
So since my system adds up penalty's and gives a total score, where as the other mentioned systems do not, it is an obvious side effect that HCA will give a preference to stones with larger diameters for their weight and this means that there will be a slight preference directed to shallower stones. However per se my personal preferences are not that far from Tolkowsky - adjusted slightly shallower because it is what ladies want: i.e.

Size does count.
Bigger is better!

In the future when making comments about HCA I hope the experts and prosumers here will take this into account.

for further original 10 year old commentary http://www.diamond-cut.com.au/14_spread.htm

I might add that my system is the only one that also relates the gridle thickness risk of chipping to other individual proportion risks such as shallow crown angle. So just going shallower and spreadier with those two factors is more than compensated for by chipping risk penalty.
Regarding this comment on that page
We choose to give a zero penalty for a diamond with a 40° pavilion, 32.5° crown, 58% table and a 1% girdle, diamonds with better spreads also rate zero. The worst score of 2.0 is given to a diamond with a 10% girdle, 40° crown, 54% table and 43° pavilion. Stones with even worse spreads are not further penalized.
I have since adjusted the spread factor, but since this site is an original paper - I do not like to edit it. I do not have the current mimimum with me on vacation, but it is more like 40.5 and 32.5.
Click to expand...


Those are good points - I didn't consider them! Those are things we might *know* to look for, but like you say I can't think of another tool that takes them into consideration automatically.
I would add null checking and validation to that - I know other tools may permit consideration of physically impossible proportions, HCA will tell you off if you've entered something bizarre..
 
Hi Paul and Gary,

Many thanks for taking the time to reply to my post (Yssie and others as well).

Paul, your explanation of the three evaluating factors were interesting and succinct, also confirming all three evaluating factors HCA, AGS and GIA are looking for diamond 'brightness' being essentially the same goal.  However, I don't think consumers understand that, and verbatim they will quote the HCA as 'oh, I got a VG for fire but an Ex for Scintillation, what do you think, will the diamond still have fire do you think?'

Gary, you've created a tool that has helped many people identify a diamond on the Internet. That's wonderful and hugely commendable. However the HCA has also created an absolute 'fear mongering' amongst PS consumers where they completely obsess over leakage, and posters suggest returning AGS0, GIA ex ex ex and diamonds from very reputable Jewellers.

Spread is nice, but the number one character consumers want is a 'sparkly' diamond.  I understand a lot of long time posters here are heavily invested in the HCA and no doubt have beautiful diamonds as a result.  What concerns me is seeing new consumers second guess their decision of an AGS0 or a GIA ex or stone because it scores above 2 on the HCA and either do not bother with it at all or are advised to return it and are fearful of 'leakage'.

It's great you've adjusted the HCA theoretically, but no one on this forum knows that. They still follow what exists on this site. Also you mentioned your preference is not too far off from Towolsky so why would a 42.2 pav 33.5 crown be rejected at 2.5 on the HCA.  This could very well be an absolutely optimally performing diamond and is inversely proportional to Towolsky dimensions

A simple graph from octonus shows three variables, and most of us can read a graph. However most PS consumers choose not to and rely on other members on the forum or, simply put 4 numbers into the HCA.  The graphs from octonus indicate that the following proportions would be AGS0 and GIA ex candidates simultaneously and yet would be rejected on the HCA. For example:

41.2 pavilion 33.5 crown HCA score 2.3 
41.2 pavilion 34 crown HCA score 2.8
41 pavilion 35 crown HCA score 2.1 
41.4 pavilion 33 crown HCA score 3

None of these ideal/ex candidates would even be investigated further.

I've also looked at dimensions that the AGA calculator would grade 1a ideal (what the creator said was even more specific than AGS0) yet these combinations also failed the HCA.

Consumers and posters here believe so strongly in the HCA in predicting fire, scintillation and brilliance.  They even interpret the graph that is generated 'literally'.  Many of these new posters may only ever buy one diamond in their entire life.  The average consumer knows limited information about diamonds and they will follow quite blindly what others say, where I can see confirmation bias occuring as well.

Disregarding diamonds that both AGS and GIA have evaluated and could likely be considered ideal and excellent simultaneously and most likely very beautiful and bright diamonds just seems really really unfortunate. Giving up colour in a stone for something that potentially cannot even be seen like the difference between an IF or VVS1 or even just the 'belief' in that is also sad.
 
From a consumer/lay researcher standpoint I really value HCA after seeing the difference IRL between a 35.5/42.1/62.3 3x and a 33/40.8/60.7 3X, both with a 56% table which were offered as a pair. The steep/deep, although beautiful, had a lot less sparkle, and was readily distinguishable from it's partner. There was also a triangular zone under the table which did not return light well when viewed from the side. The other stone did not exhibit this dark area. The HCA for the steep/deep was 4.4 and the other .07.

Stereo vision did not compensate for the difference. Diamonds are not always viewed from the top down, and it does not mask the difference in the number of flashes of light which can be seen when the stones are moved from side to side while next to each other. To account for bias/dominance I switched each stone from the L to R position several times.

My VG cut engagement stone has a lot more life than the 35.5/42.1 3X even with it's misaligned crown and pavilion facets. Oh if only I had found this site at that time!

I'd wager that outside the lab, the majority of the time diamonds are viewed from an angle. I always have to adjust my ring in order to view it from the top down, and to get a proper top down view of someone else's diamond I would have to have to adjust it as well. Pretty cheeky to grab someone's hand and have a good long stare :lickout: . So, keeping that in mind, steep/deep diamonds show darker and less sparkly, with a dead zone, in all but top down bright light situations. Just my own observations.
 
EDIT: Sorry I made a typo, I meant a 41.2 pav with a 33.5 crown, would be inversely proportional to tolwolsky and score a 2.5 on the HCA.

That is not a steep/deep stone nor the other examples I mentioned scoring between 2-3 on the HCA.

Of course an exceptionally steep stone causes a nailhead as well as a super shallow stone will be glassy or have a fisheye.
No doubt a 42.1/35.5 was deep. I did not mean to give 42.2/33.5 as an example, 41.2/33.5 was the example.

If you thought a 41.2/33.5 triple ex cut was not sparkly I would be super surprised.
 
Not sure what is the problem. It is a rejection tool based on averaged and rounded numbers, not a selection tool, the aim of it is to reduce the number of candidates for further analysis, with ASET/IS image etc. If those are available, HCA is not really required.
 
Because you reject a lot of potentially ideal performers, but spend time analysing and giving a chance to potentially many fair and good performers. Doesn't seem right on a consumer forum to not give the other potentially ideal performers a chance to be investigated further, which I've repeated several times in this thread. The diamonds between 2-3 on the HCA in particular seem unfairly weeded out.
 
And what do you mean by giving chance to fair/good performers? Based on what?

If you want to investigate those, it is your right.
 
Stone cold, if you are going to respond please at least take the courtesy to read the entire post and links provided so you don't just comment randomly. If you read the thread you would not need to ask this question and waste time.

If you read the posts, you will understand the reasoning - it is validated and logical (hopefully not too complicated to follow), if you take time to do some research, understand the grading labs, the evolution of the GIA and AGS cut standards, and the fact that Gary Holloway has updated the parameters to himself but not in the HCA tool you use on PS.

He has also said himself the HCA is too strict in previous posts, and up to 3 is reasonable to consider as a score on the HCA.

I don't want to change the ethos of PS, I'm not going to write 15000 posts saying the same thing, just be wary, the price differentiation for vendors between a VG and an Ex stone hugely changes their profit margins.
 
2023|1343540575|3242385 said:
Stone cold, if you are going to respond please at least take the courtesy to read the entire post and links provided so you don't just comment randomly. If you read the thread you would not need to ask this question and waste time.

If you read the posts, you will understand the reasoning - it is validated and logical (hopefully not too complicated to follow), if you take time to do some research, understand the grading labs, the evolution of the GIA and AGS cut standards, and the fact that Gary Holloway has updated the parameters to himself but not in the HCA tool you use on PS.

He has also said himself the HCA is too strict in previous posts, and up to 3 is reasonable to consider as a score on the HCA.

I don't want to change the ethos of PS, I'm not going to write 15000 posts saying the same thing, just be wary, the price differentiation for vendors between a VG and an Ex stone hugely changes their profit margins.
Click to expand...

2023 I agree there can be stones in the 2-3 range worth considering. If they are top symmetry, if there is no painting or digging.
When I first began working on the HCA data there were way fewer stones with really good optical symmetry, so given the choice of slightly deeper or slightly shallower in my experience shallower works better if there are sym deviations.
Marge_C confirms this.
 
Gary THANK YOU so so much for your reply. Yes, it is exactly those diamonds with excellent graded symmetry excellent graded cut that I had hoped would be investigated further in the HCA 2-3 range.

I found this link below, and even with at least half a dozen diamond experts/cutters it took a lady four pages of heavily referenced scientific data to feel comfortable with a 34/41 diamond because she had been so fearful of going outside PriceScope considered norms. This was crazy! Everyone had to literally convince her with studies, images, computer generated light simulations etc, that she was getting an amazing diamonds, but she was absolutely terrified! Luckily she kept the diamond. Jonathan W (rhino) also discussed GOG HnA diamonds having pavilion angles of 41, michealgem and strmdr similarly liking the 41 pav. Serg provided images/stmdr virtual idealscopes another actual light reflections, that how intense the 'fear mongering' can get, that four pages of posting by experts was needed for this lady.

[URL='https://www.pricescope.com/community/threads/41-degree-pavillion-angle.65301/page-11']https://www.pricescope.com/community/threads/41-degree-pavillion-angle.65301/page-11[/URL]

Your words are immensely powerful in this community Gary, unbelievably so, that people read literally into your fire/scintillation/brightness graphs as characteristics of a diamond.

I appreciate your response greatly. Thank you.
Enjoy your holiday!
 
Also the HCA from MaggieC was a 4.4, and a proportion not within the ranges I was referring to in the previous posts. :))

I also noticed lated a 41/34 stone popped up later in a post, and everyone loved it, with you also saying 'it doesn't get more perfect than that'!
 
Yssie|1343366413|3241203 said:
2023|1343364598|3241186 said:
My question is why diamonds that fit parameters of an AGS and GIA ideal and ex candidate are so swiftly disregarded, just reading a simple graph, off octonus, where the results are very obviously correlated that a child could see a trend in AGS and GIA ideal and ex yet these results are discarded by the HCA.

The HCA is a very simple tool with only 4 variables, the graphs are very simple with only 3 variables...they should relate to each other more. The GIA and AGS graphs relate.
Click to expand...


... the three systems aren't supposed to mirror each other, they aren't designed to mirror each other, and that's perfectly okay!

I'm kinda getting the feeling that the bigger issue is really What Is PS For. In MY opinion the goal is to steer new buyers toward making "educated purchases", unless they explicitly state otherwise. And MY definition of an "educated purchase" is not to just find a stone that sits in the middle of the AGS/GIA/HCA/AGA/whatever venn diagram, it's to encourage buyers to figure out what THEY like and help them find that. Sometimes that means going for the middle, sometimes it doesn't. And in MY opinion HCA does not need to have more overlap with GIA and AGS to be a useful tool. Obviously not everyone shares "MY" opinion!

Why I feel that way:
1. Stones in the middle aren't prettier, better performers, more nuanced, livelier than stones not in the middle. There are lots of flavours of lovely stones.
2. If you limit yourself to that small scope you're excluding a LOT of nice stones. This can be insignificant if you're looking in a popular size/clarity/colour set, but if there aren't a ton of stones to choose from you've just eliminated a huge number of candidates.
3. You're limiting yourself to one or two specific flavours. Some people (me) really like butter pecan, better than vanilla/chocolate/strawberry, and some people (me) don't particularly care for chocolate.
4. Youre missing out on any deals that might come about b/c of slightly unorthodox proportions.
5. GIA, AGSL, and HCA don't consider optical symmetry, only facet meet symmetry. So if symmetry of facet patterning (H&A and whatnot) is important to you you're still going to have to do some scouting on your own.

That said, it's
1. An effective way to find a nice stone, if there are enough left to choose from after culling everything else (ignoring the optical symmetry issue)
2. An efficient way to find a nice stone, if there are enough left to choose from after culling everything else (ignoring the optical symmetry issue)
So if a certain buyer's priorities are to get a good one as quickly as possible, even if it costs a bit more, it's a useful strategy.

It's usually pretty clear in the OP whether a buyer just wants a good one this week or whether he/she is interested in delving into the details.


To the specific issue here - that HCA is hard on steeper pavs is a well-known and well-documented issue, but you seem to have deeper misgivings. What's your suggestion?
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:appl: :appl: :appl: Thank you Yssie!! You have said all that I wish I could have said but couldn't articulate!

2023You have expressed your discontent with the HCA over and over, in fact most of your 142 posts have been about it, yet you still seem to be confused about it's uses and limitations. NO ONE has ever claimed that the HCA wouldn't miss a potentially beautiful stone! However, when shopping site unseen it remains one of the most helpful tools we have. I would rather miss a few beautiful stones in a long list and be assured that my short list was full of them, then to play guessing games with GIAs broad range of EX cuts. As Yssie already pointed out, to follow your suggestion of increasing the cutoff will also result in many more dogs passing as well. I don't feel that the HCA is broken, it is doing exactly what it was designed to do. So far it appears that your only criteria for a gorgeous diamond be that it carries a GIA EX cut grade. Many here disagree. Possibly as you continue to gather information and explore you will come to agree as well. ;))
 
Christina, I think you misunderstand me completely.

I have never said that a diamond must be a triple ex GIA to be beautiful, I know that many PSers go outside those areas very happily and find great diamonds they like that are not GIA ex ex ex. I have also never said a diamond must meet simultaneously AGS0 standards, GIAexexex and AGA 1a standards to be beautiful either, but in this category, there may be a comfort factor. The AGA calculator is even stricter than AGS0, and diamonds that grade 1a can still get between 2-3 on the HCA.

My point was consumers who don't know as much about diamonds (as say yourself) have been concerned about AGS0 stones and HnA diamonds and I used examples of GIA ex stones that can fall into the AGS category and AGS0 stones in all my examples. If you read my posts I never ever suggested going into the depths of 'broad GIA ex' as i believe that would freak everyone out. All examples were both AGS ideal and Ex candidates. I know people trust the HCA a lot and have bought a lot of diamonds based on that, I am not disputing that.

The mentality of so many new posters, the literal interpretation of the HCA... well I have a VG for brightness but a Ex for fire...fear of leakage, fear of so many things, yet not logically looking at a diamond with potentially ideal proportions because it gets between 2-3 on the HCA, where many ideal candidates sit, that's why I wrote this thread. Consumers should know this.

Gypsy suggested an amazing stone to someone, 41.2/33.5 and the stone i think will be amazing yet is 2.4 HCA.
Another poster looked at two stones and chose the HCA at 2.7 as it was brighter and stood out more to her and her husband.

HCA scores between 2-3 can be more ideal cut diamonds than many that get HCA scores of 0.5. And Gary H also said in this post the HCA cut off for shallowness Is too low, and he says so himself in this thread. On top of that I was simply saying don't disregard a stone between 2-3 and that it is worthy of further investigation and again Gary H has said that in previous posts and again in this thread too.

I don't know if you read the whole thread, but I referenced points and I was succinct. I am not saying a VG stone is a terrible diamond (or is a dog as you say here) and if people feel that's what I am saying and getting emotional about that, I believe that is their own insecurities as I never ever said that.

You mentioned in another thread to start a new post and see if Gary H would reply, I simply followed your advice.
 
2023|1343619304|3242752 said:
Christina, I think you misunderstand me completely.

I have never said that a diamond must be a triple ex GIA to be beautiful, I know that many PSers go outside those areas very happily and find great diamonds they like that are not GIA ex ex ex. I have also never said a diamond must meet AGS0 standards, GIAexexex and AGA 1a standards to be beautiful either, but in this category, there may be a comfort factor. The AGA calculator is even stricter than AGS0, and diamonds that grade 1a can still get between 2-3 on the HCA.

My point was consumers who don't know as much about diamonds (as say yourself) have been concerned about AGS0 stones and HnA diamonds and I used examples of GIA ex stones that can fall into the AGS category and AGS0 stones in all my examples. If you read my posts I never ever suggested going into the depths of 'broad GIA ex' as i believe that would freak everyone out. All examples were both AGS ideal and Ex candidates. I know people trust the HCA a lot and have bought a lot of diamonds based on that, I am not disputing that.

The mentality of so many new posters, the literal interpretation of the HCA... well I have a VG for brightness but a Ex for fire...fear of leakage, fear of so many things, yet not logically looking at a diamond with potentially ideal proportions because it gets between 2-3 on the HCA, where many ideal candidates sit, that's why I wrote this thread. Consumers should know this.

Gypsy suggested an amazing stone to someone, 41.2/33.5 and the stone i think will be amazing yet is 2.4 HCA.
Another poster looked at two stones and chose the HCA at 2.7 as it was brighter and stood out more to her and her husband.

HCA scores between 2-3 can be more ideal cut diamonds than many that get HCA scores of 0.5. And Gary H also said in this post the HCA cut off for shallowness Is too low, and he says so himself in this thread. On top of that I was simply saying don't disregard a stone between 2-3 and that it is worthy of further investigation and again Gary H has said that in previous posts and again in this thread too.

I don't know if you read the whole thread, but I referenced points and I was succinct. I am not saying a VG stone is a terrible diamond and if people feel that's what I am saying and getting emotional about that, I believe that is their own insecurities as I never ever said that.
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Garry said no such thing. I think you need to reread what he wrote.

The HCA doesn't disregard stones that score between 2-3 all together either, it states that it's worth buying if the price is right, and most PSers would suggest accumulating more information on them. NO one has ever suggested that a person purchase or not purchase a stone based on the HCA alone. You will see thread after thread advising that the OP request IS images as they are a better indicator of LP.

The HCA is a predictor of LP not cut grade.


Many diamonds with excellent scores may not be traditional 'ideal cuts' but we believe their value will rise once the GIA establishes its cut standard.
This system is being continually fine-tuned and results may change without notice.

Garry Holloway FGAA DDT

The only emotion I have toward the subject is frustration. As often as this tool is explained to you, you still fail to understand it. You appear to take the information presented to you and twist it to meet your logic,regardless of the intent in which it was presented.

It's clear you love your steep/deep and that's great, but it doesn't change the fact that most will have leakage as the HCA predicts.
 
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