Two More Questions about the HCA

[La Jolla]

Having read through older postings on the subject, I now have a much better grasp of the strengths and limitations of the HCA. Still, I have a couple of questions.

First is this. I get it that, below 2, a lower HCA score isn’t necessarily better. They are considered all on equal ground. Some indeed prefer a score between 1 and 2 to a score below 1. Question: How about above 2? Is 2.4 better than 5.2? Or are scores above 2 similarly regarded as all equal?

A second question, more like a comment: I do see how some newbies could easily be led into misinterpreting the HCA, and here’s the problem I see. The HCA assigns grades of Excellent, Very Good, Good, Fair and Poor to each of four criteria: A diamond’s Light Return, Fire, Scintillation and Spread. A stone that scores 0.7 will probably receive Excellent on all four criteria, whereas one that scores 1.9 will probably receive "only" Very Goods.

Well, one would be forgiven for thinking that Excellent is “better” than Very Good! The grading system seems to build a certain ambiguity into the interpretation of the HCA.

Anyway, once you know what not to do, then there’s no problem. But like many others on this forum, I spent several days going through online diamond collections, running them through the HCA, and rejecting all the diamonds that scored 1.4 – 1.9, thinking that the ones that scored below 1.0 were superior. And I''m sure others will come along and do the same. Perhaps something for Garry to think about?

 

[stone-cold11]

Huh? Usage of the HCA is already explained in the usage warning and information.

 

[Lorelei]

Date: 9/26/2009 10:23:15 AM
Author:La Jolla
Having read through older postings on the subject, I now have a much better grasp of the strengths and limitations of the HCA. Still, I have a couple of questions.

First is this. I get it that, below 2, a lower HCA score isn’t necessarily better. They are considered all on equal ground. Some indeed prefer a score between 1 and 2 to a score below 1. Question: How about above 2? Is 2.4 better than 5.2? Or are scores above 2 similarly regarded as all equal? This depends, a 2.4 could well be better than a 5.2, some diamonds which score between 2 and 3 with top symmetry can also be good choices.

A second question, more like a comment: I do see how some newbies could easily be led into misinterpreting the HCA, and here’s the problem I see. The HCA assigns grades of Excellent, Very Good, Good, Fair and Poor to each of four criteria: A diamond’s Light Return, Fire, Scintillation and Spread. A stone that scores 0.7 will probably receive Excellent on all four criteria, whereas one that scores 1.9 will probably receive 'only' Very Goods. Yes this can happen that newbies misinterpret the use of the HCA, often they think the HCA is used to select diamonds instead of for rejection - although it does state clearly on the usage instructions that it is not used to choose a diamond, this still comes up. Out of interest, diamonds which measure 60.3% or less for depth will get Ex for spread. The HCA cannot see the diamond and scores are only predictive and not actual.

Well, one would be forgiven for thinking that Excellent is “better” than Very Good! The grading system seems to build a certain ambiguity into the interpretation of the HCA.

Anyway, once you know what not to do, then there’s no problem. But like many others on this forum, I spent several days going through online diamond collections, running them through the HCA, and rejecting all the diamonds that scored 1.4 – 1.9, thinking that the ones that scored below 1.0 were superior. And I'm sure others will come along and do the same. Perhaps something for Garry to think about? I am sure he will see this thread but I personally think the explanations on how to use the HCA are clear, however any feedback on the HCA is always useful to Garry I am sure.

For example, above the HCA it does state clearly that this tool is used for rejectionhere. and here.

The quotes below are from both links concerning the use of the HCA.


" HCA gets no info on symmetry, polish and minor facets; use it only to reject likely bad performing diamonds to narrow down your final selection. Ideal-Scope images and independent appraisers can help after that."

Also -

"HCA is only for round diamonds with known Table, Crown, Pavilion and Total Depth inputs; it has no idea about the diamonds symmetry, poor minor facet proportions or various cutters tricks.

Use it to reject known poor performers and narrow down your selection. Please do not use it for final selection; it will not replace an analysis with an Ideal-scope and/or an independent appraisal."

 

[La Jolla]

Thanks for your clarifications, Lorelei.

You wrote: “A 2.4 could well be better than a 5.2, some diamonds which score between 2 and 3 with top symmetry can also be good choices.”

That’s something I''m learning for the first time. For those of us who don’t have ready access to I-S or ASET images, the HCA is the next best thing to getting an approximate sense of an online diamond’s potential. And sometimes you come across a stone at a really great price that scores 2-point-something, but you automatically reject it because, as many here have written – perhaps most recently Allison Dewey of Whiteflash on Sept. 23 -- “those under 2 pass, those above 2 don''t pass.”

Unless I''m mistaken, I hear you saying: Don''t reject a 2.4 or a 2.8 out of hand.

You wrote: “Often [newbies] think the HCA is used to select diamonds instead of for rejection - although it does state clearly on the usage instructions that it is not used to choose a diamond, this still comes up.”

That''s a point Stone-cold11 also made. The thing is, with all due respect, it’s probably easier for older hands to say this. As a newbie visiting the HCA (or any other) webpage for the first time, you’re being bombarded by so much new information which you’re trying to assimilate that it’s easy to miss something that you read only once. I mean, my college education didn’t help me avoid the error! And it''s happened to many others. It’s not a question of being dumb or careless. The do’s and don’ts of the HCA could probably be presented a little more clearly, and perhaps even repeated.

But what a great tool and a great service from Garry.

 

[strmrdr]

Date: 9/26/2009 10:23:15 AM
Author:La Jolla

I spent several days going through online diamond collections, running them through the HCA, and rejecting all the diamonds that scored 1.4 – 1.9, thinking that the ones that scored below 1.0 were superior. And I''m sure others will come along and do the same. Perhaps something for Garry to think about?

That does seem to be a common problem.
Do you have any ideas on how PS can make it more clear on the HCA page?
PS always strives to provide the best information as clearly as possible and feedback from users is the best way to do that.

 

[stone-cold11]

If no IS/ASET images is present, go safe, which is why the cut off that Garry used is a score of 2.

 

[Regular Guy]

Hi LaJolla,

I appreciate your comments, and have said asked/talked about with others in much the same way before...see here, for example.

I think that while the 0 - 2 area is compromised by "being close to the axis of measurement, or fulcrum," otherwise...I pretty much disagree with my friends, and I think its clear from Garry's HCA development website that essentially...the HCA Is a linearly oriented tool...and to my mind...although...yes...he has qualified you can frequently, reasonably, consider diamonds scoring 2 - 3, and that he might revisit re-scoring HCA to even include these...to the extent the HCA "works," you must consider a 2.x significantly "better" than a 5. anything...yes.

(edited to add...in the page 3 warnings Garry provides with the HCA, after executing a proportion "check," among other things, he does point to symmetry variations as at least one of the things that will help a diamond that would otherwise receive a lower score, i.e., between 2 - 3, also be considered "excellent," as...where he says: "Stones near the center of the red region (the lowest scores) are least affected by symmetry variations. Alternatively hearts and arrows diamonds, which have excellent optical symmetry, but often HCA scores around 2, may out-perform diamonds with lesser symmetry and lower HCA scores.")

The "cut rank" tool on the "search by cut" database uses the very cut-offs of 0-2, 2 - 4, 4 -6 etc., to differentiate between excellent, very good, good, etc, based on these strata that the HCA calculates.

Although I don't have any clinical gravitas to speak of with respect to any of this, to the extent the HCA is compelling, and I've been obsessed enough to be interested in it for some time...this aspect of the HCA does seem clear enough to me.

 

[Regular Guy]

While I'm here, I'll raise a consumer challenge, prompted by previous and recent discussions. Stone cold says:

Date: 9/26/2009 1:28:04 PM
Author: Stone-cold11
If no IS/ASET images is present, go safe, which is why the cut off that Garry used is a score of 2.

Also, previously and recently, both Stone Cold and Lorelei have cautioned to check an AGS Platinum measured diamond for light leakage, since you don't have a lock on getting a good performer with a zero.

You don't want to be afraid of measurement error, but you do want to know how to analyze where your measurement error is coming from.

The question becomes, when given a choice of two diamonds...one with what seems to be a better IS, and the other uniquely getting the 0...all other things being equal...which do you prefer?

And...is this question the same as...where is the measurement error?

If you presume, as I think Garry has allowed, that a 0 more or less trumps an HCA 0 - 2, you might agree this post is actually completely, linearly, consistent with, and working in concert with this thread....if you will allow that AGS's measured 0 is actually intending to accomplish more accurately what the HCA 0 - 2 tries to do with less information.

 

[Lorelei]

Date: 9/26/2009 11:29:52 AM
Author: Lorelei

Date: 9/26/2009 10:23:15 AM
Author:Lorelei
Yes this can happen that newbies misinterpret the use of the HCA, often they think the HCA is used to select diamonds instead of for rejection - although it does state clearly on the usage instructions that it is not used to choose a diamond, this still comes up. Out of interest, diamonds which measure 60.3% or less for depth will get Ex for spread. The HCA cannot see the diamond and scores are only predictive and not actual.

La Jolla
Anyway, once you know what not to do, then there’s no problem. But like many others on this forum, I spent several days going through online diamond collections, running them through the HCA, and rejecting all the diamonds that scored 1.4 – 1.9, thinking that the ones that scored below 1.0 were superior. And I'm sure others will come along and do the same. Perhaps something for Garry to think about?

Lorelei
I am sure he will see this thread but I personally think the explanations on how to use the HCA are clear, however any feedback on the HCA is always useful to Garry I am sure.

Date: 9/26/2009 1:13:39 PM
Author: La Jolla

Unless I'm mistaken, I hear you The thing is, with all due respect, it’s probably easier for older hands to say this. As a newbie visiting the HCA (or any other) webpage for the first time, you’re being bombarded by so much new information which you’re trying to assimilate that it’s easy to miss something that you read only once. I mean, my college education didn’t help me avoid the error! And it's happened to many others. It’s not a question of being dumb or careless. The do’s and don’ts of the HCA could probably be presented a little more clearly, and perhaps even repeated.

But what a great tool and a great service from Garry.

As mentioned above this does come up a lot, maybe a clear, bold, banner type heading as an intro to the HCA on all its pages saying that it is not to be used for selection etc might be helpful?