Understanding the HCA better

[oldminer]

The HCA is based on the following scale:
"0-2 Excellent, 2-4 Very Good, 4-6 Good, 6-8 Fair, and 8-10 Poor"

I am often asked hairsplitting questions such as is a 1.0HCA going to be "better" than a 1.2HCA?" In all truth I likely cannot see the difference with my naked eyes, but that does not mean there is no absolute difference measureable in some technical way. I also get asked about a stone falling close to the 2.0HCA reading and how it would compare to a 0.7HCA stone. Again, possibly I might see the difference, but both would rate "Excellent" under the HCA strategy of grading categories. Would you all consider such diamonds both equally excellent or do you grasp the concept of a range of diamonds slightly different from one another all being in the excellent pool?

What is the highest (worst) numerical score for an AGS000 anyone ever saw? What is the highest numerical score for a GIAXXX anyone has ever seen? Such questions are often being asked and I truthfully don't have any exact experience with the limits of the HCA results. Can there be a diamond AGS000or GIAXXX with an HCA above 2.0? 2.1, 2.2--3.0 Where is the limit?

 

[Karl_K]

Worst ags0 over 5 hca
Worst gia EX over 7 hca

Gypsy has a list of wonky ags0 and gia ex she posted just a few days ago.
Maybe someone can link I don't have time to search.

 

[Karl_K]

Would you all consider such diamonds both equally excellent or do you grasp the concept of a range of diamonds slightly different from one another all being in the excellent pool?

In any diamond grading systems there will be a variety of personalities in the top grade for the simple reason that there is no one single best combination.
The best system will limit it to personality differences and not quality differences.
Such a system does not exist which is why I like to use a combination of technology and weigh what each is telling me.
The final answer is the person living with the diamond in their own environment for at least a few days if not years. Since it is pretty much impossible to do that with multiple stones you have to select the one that has the most potential to please the wearer.

 

[Rhino]

The HCA is based on the following scale:
"0-2 Excellent, 2-4 Very Good, 4-6 Good, 6-8 Fair, and 8-10 Poor"

I am often asked hairsplitting questions such as is a 1.0HCA going to be "better" than a 1.2HCA?" In all truth I likely cannot see the difference with my naked eyes, but that does not mean there is no absolute difference measureable in some technical way. I also get asked about a stone falling close to the 2.0HCA reading and how it would compare to a 0.7HCA stone. Again, possibly I might see the difference, but both would rate "Excellent" under the HCA strategy of grading categories. Would you all consider such diamonds both equally excellent or do you grasp the concept of a range of diamonds slightly different from one another all being in the excellent pool?
What is the highest (worst) numerical score for an AGS000 anyone ever saw? What is the highest numerical score for a GIAXXX anyone has ever seen? Such questions are often being asked and I truthfully don't have any exact experience with the limits of the HCA results. Can there be a diamond AGS000or GIAXXX with an HCA above 2.0? 2.1, 2.2--3.0 Where is the limit?

Quite honestly Dave there are too many variables the HCA doesn't answer that can make or break a diamond.

No account for lower half length.
No account for painting/digging.
Only an average of 17 out of 57 facets.
No account for index/azimuth angles.
Assumes precise optical symmetry (which most diamonds do not have).

Fact is you can have two diamonds scoring the same exact score and one be an AGS 0 and one be an AGS 2 or 3 in light performance.

We have fielded so many questions on this I've written an article with an accompanying video relating to the subject. I know Garry made the HCA with the best of intentions and with a heart to help consumers but sometimes I think it causes more confusion than good. IMO the vendor should be able to provide the details the consumer is looking for as there is no crystal ball that genuinely & accurately predicts the light performance and appearance of a round brilliant. Live examination is crucial for the consumer seeking answers.

 

[oldminer]

I think the responses have a great deal of importance to those consumers learning about diamonds on Pricescope. To me, the answers are eye opening at the very least. Maybe this thread can be listed in the important topics section so it can be more readily found. It would be a definite benefit to consumers to better understand the HCA for its basic merits and its potential for quite a bit of mis-direction. THX

 

[denverappraiser]

People are now using the HCA for completely different purposes than what was originally intended. They’re STARTING with a GIA-excellent cut grade and are hoping to subdivide that into differing grades of excellence. Presumably the lower the number the better. It’s intuitive, it’s free, and at least it’s doing SOMETHING in the face of dozens or hundreds of what seem like identical stones.

As much as I’m inclined to criticize the GIA-x grade as overly broad, they have a valid point that taste plays a significant component and I definitely do not agree that a GIAx/HCA0.9 is ‘better’ than a GIAx/HCA3.0. Some people may like it better and others won’t but the extra step in grading has not helped. It certainly has no impact on price and, outside of Pricescope, it has no impact on saleability.

 

[kenny]

What I think is lost on pros is the the HCA and Idealscope are unique and ingenious tools for the masses.
Sure, you pros know so much you don't need them to spot the various versions of good cut.
Sure, your advanced knowledge can locate good diamonds that are rejected by these tools.
Sure, your expertise can even pick the best of the best from several rounds that passed both HCA and IS with flying colors.
While it's always possible to get a pencil a little sharper, how sharp does it need to be?

HCA and IS remind me of Pareto Principle of finding the key 20% of work that results in 80% of the benefit.
http://en.wikipedia.org/wiki/Pareto_principle

Pros don't need these tools.
Noobs do.

The majority of diamond buyers will buy only an engagement diamond.
They are never going to learn a fraction of what you guys know.
HCA and IS is a god-send for these people because it is a simple quick no-brainer way to be assured of locating a well cut round.

Nobody is asking you guys to depend on HCA and IS to run your businesses.
Please do not bad mouth these tools for use by noobs.
Imagine the crappy diamonds noobs would be stuck with (at the majority of diamond sellers) without the HCA and IS.

Reject rounds that score over 2.0.
Get IS pic for those scoring under 2.0, since Crown and Pavilion angles input to the HCA are averages of 8 numbers, and with averages all 8 may be the same(good) , but some individual CAs or Pas may be high and low but average out well (bad), and an IS pic will reveal this.
Tell this to your customers, David.

 

[denverappraiser]

Kenny,

I can't speak for Dave or Jonathan and I'm not picking on the Idealscope at all. I"m not even picking on the HCA as it was intended, a tool for separating out a mass of stones sight unseen where all you have is a few angles and dimensions extracted from lab reports. I'm pointing out that this isn't what people are using it for. You're a consumer advisor who routinely gives recommendations on stones using very limited data. In YOUR opinion, is a GIAx with a 0.9 HCA better than an otherwise identical GIAx with a 2.0? Why? The GIA-x cut grade is already exactly your requested quick no-brainer filter on modern round brilliants and people are using it for exactly that purpose. Does the HCA narrow that focus or just confuse it?

 

[kenny]

Kenny,

I can't speak for Dave or Jonathan and I'm not picking on the Idealscope at all. I"m not even picking on the HCA as it was intended, a tool for separating out a mass of stones sight unseen where all you have is a few angles and dimensions extracted from lab reports. I'm pointing out that this isn't what people are using it for. You're a consumer advisor who routinely gives recommendations on stones using very limited data. In YOUR opinion, is a GIAx with a 0.9 HCA better than an otherwise identical GIAx with a 2.0? Why? The GIA-x cut grade is already exactly your requested quick no-brainer filter on modern round brilliants and people are using it for exactly that purpose. Does the HCA narrow that focus or just confuse it?

For some reason you are asking a question that has been answered a zillions times here.

Reject if over 2.
Get IS pic if under 2.

Easy Peasy.

When you get noobs who ask about a 0.9 vs. a 1.7 repeat the above.
Over and over and over.

That's the 20% that yields the 80%.

Why 2.0?
You gotta draw a line somewhere, just like a 17 year old can't vote a day before her 18th birthday.

 

[Karl_K]

Kenny does have a very good point.
IS is over HCA and GIA EX grade, but when an IS isn't available I will use HCA over GIA EX.
IS/HCA combo is a quick way to a reasonably high performance RB quickly.

 

[John P]

As it relates to non-pros - I am largely with Kenny and Karl on this one.

 

[denverappraiser]

I’m certainly aware of the traditional advice given here but it seemed good to get it injected into this thread and the easiest way was to ask the question. It's sort of a Socratic thing.

So, in terms of evaluating GIAx’s, the HCA is a Boolean decision. Over 2 no, under 2 maybe.
How much over or how much under doesn’t change the answer, correct?

Does the same rule apply on evaluating other GIA grades? I presume that GIA-g, GIA-f and GIA-p are all deal killers on their own right so the HCA doesn't add anything so the key point may center on evaluationg GIA-vg's. Is it helpful? How?

 

[Karl_K]

Does the same rule apply on evaluating other GIA grades? I presume that GIA-g, GIA-f and GIA-p are all deal killers on their own right so the HCA doesn't add anything so the key point may center on evaluationg GIA-vg's. Is it helpful? How?

The hca can be used to find some awesome gia VG's for pendants and earrings that GIA dings for shallow pavilion/contrast issues.
Then there are the GIA vg with painting that are better than some GIA Ex graded diamonds.
There are cases where HCA is useful with GIA VG and maybe lower, I haven't bothered with the lower grades searching for acceptable combos.

 

[denverappraiser]

Does the same rule apply on evaluating other GIA grades? I presume that GIA-g, GIA-f and GIA-p are all deal killers on their own right so the HCA doesn't add anything so the key point may center on evaluationg GIA-vg's. Is it helpful? How?

The hca can be used to find some awesome gia VG's for pendants and earrings that GIA dings for shallow pavilion/contrast issues.
Then there are the GIA vg with painting that are better than some GIA Ex graded diamonds.
There are cases where HCA is useful with GIA VG and maybe lower, I haven't bothered with the lower grades searching for acceptable combos.

How do you use the HCA to evaluate painting?

Is your HCA usage rule for evaluating VG's any different from Kenny's rule on X's? <2 Maybe. >2 Reject.
If it's different, what's the difference?

 

[Karl_K]

How do you use the HCA to evaluate painting?

Is your HCA usage rule for evaluating VG's any different from Kenny's rule on X's? <2 Maybe. >2 Reject.
If it's different, what's the difference?

For painted VG's Use the hca and report notation to find them IS/ASET to evaluate them.

I think advanced users and when images are available can be less strict about 2 being the max but as Kenny said 2 is easy.
I would also use similar logic to this chart when shopping VG for pendants/earrings:


http://www.pricescope.com/wiki/diamonds/holloway-cut-advisor

The reason I say similar logic is I feel the chart is outdated and not perfect.

 

[kenny]

Is your HCA usage rule for evaluating VG's any different from Kenny's rule on X's? <2 Maybe. >2 Reject.

"Kenny's Rule"?
Huh?
I didn't make anything up.
Garry Holloway gets all the credit.

I just read and condensed the directions at Garry's HCA site:

https://www.pricescope.com/tools/hca

 

[diamonds-are-friends]

I am actually curious to know of the percentage of people who would base a purchase solely off a HCA score without additional data and what the majority of the shoppers think in this aspect.

 

[denverappraiser]

I called it Kenny’s rule because its stated as a rule you’re the one who presented it in this thread. I was trying to state it concisely but please correct me if I’m wrong. What we're discussing is slightly different from Garry’s instructions in that only one score is considered. Reject over 2, under 2 subject to further tests and consideration outside the realm of HCA.

Here’s what you said:

Reject rounds that score over 2.0.
Get IS pic for those scoring under 2.0, since Crown and Pavilion angles input to the HCA are averages of 8 numbers, and with averages all 8 may be the same(good) , but some individual CAs or Pas may be high and low but average out well (bad), and an IS pic will reveal this.

I’m not actually arguing against that. I’m just trying to get an explanation of how the HCA is used in the post-GIA cut-grade world and when used in conjunction with the GIA grade. The instructions you linked were written before the GIA grades were available and they are a bit different from your summary above. As far as I can tell, you’re calling all scores above 2 tied at ‘reject’, so Garry’s divisions here don’t apply. All stones 2+ are reject and some stones 2- will be rejected later for reasons that have nothing to do with HCA. That ends the roll of the HCA in the process. Is this not correct?

What about GIA-VG’s? Karl says to look at the details on the GIA including the comment about painting if present but I’m not seeing how that involves using the HCA as a filter. Some are worth considering based on the GIA provided painting info but that decision doesn’t seem to have anything to do with their HCA score.

GIA-VG, no painting comment and HCA 2+ = reject all? HCA under 2 = reject all? Accept all with further tests? Accept some?

GIA-VG with painting comment and HCA 2+ = reject all? HCA under 2 = consider with further tests? (this is exactly the same as with GIAx's)

Am I correct that GIA-G, F and P are automatic rejects from superb cuts selection regardless of HCA scores?

 

[Karl_K]

I am actually curious to know of the percentage of people who would base a purchase solely off a HCA score without additional data and what the majority of the shoppers think in this aspect.

No one should buy just based on the HCA score.
It is a filter to see which stones to get more information on.
In conjunction with IS/ASET it can be a quick filter to get you a reasonably high performance RB quickly.

 

[Karl_K]

Am I correct that GIA-G, F and P are automatic rejects from superb cuts selection regardless of HCA scores?

If I ever came across one with a good hca score I would consider getting more info(is/aset, scan without rounding etc.) on it.
But then again maybe not with the gia rounding if its going to cost money to get the info.
If getting the info was free then I would consider it.

 

[kenny]

I am actually curious to know of the percentage of people who would base a purchase solely off a HCA score without additional data and what the majority of the shoppers think in this aspect.

Hopefully it is zero percent, but I'll bet it's not because many people do not read directions.
If they use ONLY the HCA to select a diamond they are NOT following the HCA's directions, which tell you to get an Idealscope pic or appraiser opinion for rounds that score under 2.

Again the crown and pavilion angle input into the HCA are AVERAGES of 8 angles each.
If all 8 crown angles around the diamond really are 34.7 and all 8 pavilion angles around the diamond really are 40.8 then the IS will look perfect and symmetrical.

If not, if those angles vary all over the place but only average out to 34.8 and 40.8 then the Ideascope pic will reveal this with wonky arrows.

 

[denverappraiser]

Gentlemen,

Perhaps I have things in the wrong order. Should the methodology be to reject anything above HCA 2 regardless of what GIA says and under 2 consider it subject to further evaluation with whatever else is available? Does the GIA cut grade even apply? Presumably the Good, Fair and Poors will have received that for some reason that will turn up under the further testing category so the rejection will be for the underlying reason rather than the GIA grade itself. Even VG’s will have been downgraded for some reason that you’ll spot in the later evaluation and, as you point out, those reasons include the possibility of ones that you (and other shoppers) might call a feature not a problem.


About that ‘further testing’ step.
The options generally available are IS, ASET and other sorts of reflector images, Sarin scans, dealer observations, photomicrographs, more detailed examination of the ‘minor’ details from the lab, professional appraiser inspections, and personal observations. All but the last 2 are ‘free’ and fast. The last is obviously the trump card and applies to ALL purchases so the goal here is to make as wise a decision as possible BEFORE incurring the shipping costs and/or appraiser fees, both of which are usually non-refundable if the stone ends up being rejected. This leaves IS, ASET and similar reflectors images, certificate details, photographs, and dealer observations. Is that all correct? Usefulness of these vary a bit depending on what is available at all and there are several interconnections, like things you might see on the lab docs that raise a red flag for the dealer inspection. That’s probably a good subject for another thread but I think it goes beyond the whole HCA question so I’ll try to get back on track. Is there anything else added by the HCA BEYOND the filter of over/under 2 and the decision of whether it’s worth the effort of further consideration? For example, does the HCA answer any questions that may come up from looking at the report and/or various images?

 

[John P]

Hi all,

In terms of basic proportions HCA (AGA too) has a shallower center than AGS, and AGS has a shallower center than GIA. Resultantly the HCA gives 2.0 and under to many GIA VG and a few GIA G combos (various table sizes) such as 40.4/32.5, 40.6/31.5, 40.8/30.5.

By the way, none of those are AGS0 candidates either. But plenty of AGS0 candidates occur in GIA's VG range. Why don't we discuss that? The answer is that we do, but it's raised with far less frequency than GIA EX diamonds with >HCA 2.0 and outside of AGS0. This is a logical by-product of more and more RBs falling in the steeper range as a result of the GIA 2006 cut grade.

Should the methodology be to reject anything above HCA 2 regardless of what GIA says and under 2 consider it subject to further evaluation with whatever else is available?

Who's methodology Neil? I would never suggest to take the HCA score, the whole HCA score and nothing but the HCA score. Picking it apart as a pro, its total compass includes combos shallower than my own comfort zone allows. But if we're talking about consumers I think the HCA doth its office fairly... HCA <2.0 is like "cross at the green and not in-between." It's like "don't run with scissors." It's training wheels. It's safety. Sure I realize it dis-includes some possible winners. But it dis-includes far more "losers."

Is there anything else added by the HCA BEYOND the filter of over/under 2 and the decision of whether it’s worth the effort of further consideration? For example, does the HCA answer any questions that may come up from looking at the report and/or various images?

Not to me. There are the sub-factors of "LR, Fire, Scint and Spread" that appear on the results page. But without other details such as minor facet measurements, general indexing, variance from given averages and 3D cut precision I honestly don't consider them.

Here is a different question: Would Pricescope be better-served with - or without - the existence of the HCA?

But before anyone responds pedantically, first clothe that question in due historical context, as it relates to the early and continuing participation of non-professional enthusiasts on this site.

 

[Serg]

2mm full round cut has much higher brightness than any 6mm round GIA Ex ,AGS0, HCA below 2.
But most 6mm round cut diamonds have much higher Fire than any 2mm round full cut.

Just compare cluster from 5-7 2mm round cuts with 6mm round cut.
If we can easy see huge difference in performance for different sizes with same proportions then there is possible visible performance difference between 6mm and 7mm round cuts with same proportions. In such case which diamond has higher performance
6mm HCA 1 or 7mm HCA3?
Has 7mm AGS4 higher or less performance than 6mm AGS0?
It is very funny that you even now discuss about diamond Perfomance without any connection with diamond size.

 

[oldminer]

John; I think you have hit on a key point. The HCA and thew AGA systems are seen as useful, but might be just as well viewed as "use to be useful or not as useful as they once were" because of advancements in technology and understanding of how to cut better perfroming diamonds. As professionals we are struggling with how to advice consumers properly and I think the content of this thread shows how basic the problem is trying to mix the newest technology with older, observational and trigonometric calculation methods.

Should professionals consider revising the recommended screening steps since they may be in direct conflict with newer findings, or do we continue to use something on the order of the <2 >2 method for screening even when it may confuse rather than provide understandable advice?

Serg's point about size changing fire and brilliancy adds a further and most complex dimension to this discussion. It is something we rarely even consider, but I would suppose SWerg has investigated these assertions and I trust his knowledge on this subject.

Like so many things, we are not evolving our recommendations as rapidly as advanced technology has been entering our market. What do you think?

 

[Karl_K]

Like so many things, we are not evolving our recommendations as rapidly as advanced technology has been entering our market. What do you think?

True but....
We have the technology and knowledge to have someone take a picture of everywhere they visit in a day and design a diamond with the best compromise to look best in all those lighting conditions then have it custom cut.
However should we go there?

 

[denverappraiser]

Who's methodology Neil?

The key question of this thread, at least as I see it, is the correct usage of the HCA in today’s environment. What is it and isn't it good for? I fully admit, I very rarely use it at all, although I am fairly familiar with it. As was pointed out above, my practice, as well as that of other professionals, is rather different than most of the advisers here. When someone asks for a recommendation on the forum to choose between several stones being considered, I don’t participate. That’s the primary usage for the HCA. People want to pick a stone from the various offerings that is most likely to be satisfactory, and usually that means choosing the ‘best’ cut for a particular size/color/clarity/price combination without the benefit of anyone actually seeing the stone. That’s the methodology I’m referring to. The gurus here do this several times a day and the HCA is often quoted as part of the decision process. This process is the ‘third rail’ of Pricescope and I’m trying to keep the conversation away from unduely criticizing it and on track with how to use the HCA in a positive and useful way in conjunction with whatever other information may be available, including the GIA and AGS cut grading systems, if applicable. These systems were not available when the HCA came out or even when the above quoted instructions on using it were written but apply now to nearly every round stone being considered.

 

[Karl_K]

The biggest reason the HCA is still relevant is it answers a very important question.
Does the average pavilion angle work with the average crown angle?
Without that nothing else works.

Sure you can modify it somewhat with the lowers and painting or digging but the fundamental crown/pavilion relationship is still one of the most important things to know.