HCA Vs. GIA Why the conflict?

[longwood50]

I see that GIA has the following parameters for Excellent Cut.


GIA (Excellent) Crown Angle: 31.5° - 36° Pavilion angle: 40.6° - 41.8° Table Size: 52 - 62%

I have located a stone that has the following:

Crown Angle; 33.5
Pavilion: 40.6
Table 59
Depth 59.7

The stone cut is rated by GIA as Very Good.

When I input those into the HCA Advisor they come up with
Excellent, Excellent, Excellent, Excellent. 1.1

It appears as though all of the parameters meet GIA standards. Why the rating particularly when HCA shows it to be excellent throughout? Am I missing something?

 

[Christina...]

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 is a statement made my Garry and included on the HCA tool page. The HCA includes proportions that do not meet GIA or AGS cut guidelines for an excellent or ideal grade, however the proportions of the stone that score excellent have the potential for top performance.

People seem to be forgetting that the HCA is only a tool to help determine if a stone is a good candidate to obtain further information on. It has it's uses and limitations. Specifically to help an online shopper navigate through a huge inventory of online stones. It has never been suggested that people based their purchasing decisions based on HCA results only. We always recommend that people ask for magnified images and idealscope images of any stone they are considering, the final judge will be the purchasers own eyes.

 

[Christina...]

As it pertains to your particular stone and why it didn't receive GIAs top grade for cut, I don't know, perhaps someone else can answer that. It may be that it has brillianteering or the polish and symmetry are listed as only good? An overly thick/exremely thin girdle? Hopefully someone with more experience can help you determine that. Can you post the report?

 

[longwood50]

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 is a statement made my Garry and included on the HCA tool page. The HCA includes proportions that do not meet GIA or AGS cut guidelines for an excellent or ideal grade, however the proportions of the stone that score excellent have the potential for top performance.

People seem to be forgetting that the HCA is only a tool to help determine if a stone is a good candidate to obtain further information on. It has it's uses and limitations. Specifically to help an online shopper navigate through a huge inventory of online stones. It has never been suggested that people based their purchasing decisions based on HCA results only. We always recommend that people ask for magnified images and idealscope images of any stone they are considering, the final judge will be the purchasers own eyes.

This is the report. https://myapps.gia.edu/ReportCheckPortal/getReportData.do?&reportno=2141517547&weight=3.01

 

[Christina...]

according to facetware it should have been given an EX cut grade. Do you have the original report? Does it say anything about painting or digging? I think I've read that GIA devalues the cut grade if their is evidence this. Hopefully someone else will chime in.

 

[John P]

Christina: I got VG from Facetware at 59 40.6 33.5 80 55 3.5. The lab manual also gives the combo VG.

For the original poster: GIA excludes a number of combinations with 40.6 and lower pavilion angles from EX which other metrics approve. This is because the "center" of GIA's system falls at a higher average pavilion angle. As a result some combos at the high end will get EX where other metrics omit them.

People seem to be forgetting that the HCA is only a tool to help determine if a stone is a good candidate to obtain further information on. It has it's uses and limitations. Specifically to help an online shopper navigate through a huge inventory of online stones.

Just so. These are totally different kinds of systems. The HCA is predictive. Laboratory systems are evaluative. It is a significant fundamental difference:

The HCA was designed for the purpose of assisting online shoppers who cannot see the candidate diamonds they're considering. It takes a few broad data points and comes to a repeatable conclusion. That conclusion is based on the opinions and research of its designer, who kept it conservative on-purpose. Otherwise it wouldn't have served online communities in the popular fashion it has for over a decade. Thousands of consumers have used the HCA as a predictive device to assist in the purchase of a diamond they've never even seen. In the lifespan of this community it has been an unarguable success.

In contrast to that, GIA actually examines the diamonds for which it issues reports. So does AGSL. So do other labs (well mostly, but that's a different topic). A laboratory report gives far more details. The lab graders actually see the diamond. The lab report will also include a cut-quality assessment.

So... The HCA is predictive. The labs are evaluative.

RE this topic: The HCA prediction will not always agree with the lab evaluation. Why? Because one of the metrics is more conservative as it relates to that specific proportions combination. It's as simple as that.

Here's the good news: If someone wants to know why a lab and the HCA come to different conclusions it's easy to explain: In this case the HCA likes a broader range of 40.6 PA combinations than GIA does. If someone wants to know more details - in this case it has to do with obstruction/obscuration metrics chosen by GIA which moves the center of their range to steeper than HCA, AGA or AGSL - those details can be explained.

Another thing that bears consideration: Garry Holloway introduced the HCA several years before GIA was issuing a cut grade and before AGSL introduced performance grading. It predates those systems as a consumer-assistance tool for the kind of enthusiasts who are regulars on this site. Since it was created as a purely predictive tool - to help cut-focused folks identify candidates based on limited data without ever seeing the diamond - it's logical to me that it's more conservative than the system now used by GIA; which was designed to serve the whole planet.

In short: Yes, the HCA assessment will not always agree with the lab assessment. No big deal. You can always ask why if you'd like an explanation.

 

[diamondseeker2006]

We need to print that and frame it here, John!

 

[2023]

I was going to post that both GIA and AGS would not classify this stone as either Excellent Cut GIA or Ideal Cut AGS (with the paramters listed)...and then I saw Christinas post !!! Happy the GIA VG was clarified by John

Also, the HCA favours shallower stones (hence your great score on this), and whether thats a preference or not is something that seems to be debatable.

 

[Paul-Antwerp]

Trying to put this more in perspective, especially highlighting how HCA is a tool of some age and rather broad in nature and GIA's cut-grade is similar in approach and thus similarly broad, but not necessarily in the same average combinations, I was thinking about safety-ratings for cars.

Ten years ago, only one car reached the 5-star-level of the European NCAP-test. Now, it is considered close to a disgrace if a car only gets 4 stars: http://www.euroncap.com/latest.aspx

I see now how NCAP has added percentage-results on certain features, aspects that are clearly missing in HCA and in GIA. And I must add that even the stricter and science-based AGS-cut-grade has not evolved to include all aspects that should be part of a cut-evaluation.

On the road, even with almost all cars now being 5-star-safe, we still have far too many victims. Some observers say that this is partly due to humans taking more risks because they feel their car is so safe.

In diamonds, the ease-of-use of HCA and/or the reassurance of the world's foremost authority's cut-grade turn the assessment of a diamond's cut-grade almost into blind rubber-stamping.

In the future, labs will inevitably update their cut-grades to include new and better knowledge, and the result will be more strictness.

Live long,

 

[Christina...]

apologies I evidently entered a wrong input somewhere.

I agree with DS, Johns explanation should be posted somewhere, and I love your analogy Paul!

 

[2023]

no worries Chistina! Just confused me!

John, so great to point that out that GIA and AGS labs see the stones and grade them, as it seems to feel like people forget that part sometimes!

Like for colour grading, GIA has at least two different people compare the stone to a master set independently and then independently input the data. For the stone to pass that colour grade both assessments have to be the same. Anyhow off topic.

It is true as an online tool it has its merits for an online community of shoppers. But we've all seen examples of AGS0, HnA cuts and GIA ex cuts all get between 2-3 which a person could easily dismiss. Also there was an example of a tiffany stone too I believe, and Tiffany selects their stones again from within another subset of stones with their parameters!

As I've said already my main problem with the HCA...it seems to be gospel here to weed out stones above 2...when certified ideal cut AGS0 stones, certified ex cut GIA stones, and HnA stones can get scores between 2-3 vs very shallow stones can get scores under 1. Seems so sad these stones can get so easily dismissed.

Paul -A haha blind rubber stamping, that's going to scare everyone and we'll all be wanting Harry Winston diamonds

 

[Christina...]

2023 We actually tell people not to bother with the HCA when they are considering AGS0 stones as they have already been evaluated for light performance. GIA however does not. You've made your feelings regarding the HCA very clear, perhaps you should voice your concerns to Garry himself??? He would obviously be the most qualified to address them.

 

[Paul-Antwerp]

Paul -A haha blind rubber stamping, that's going to scare everyone and we'll all be wanting Harry Winston diamonds

I think that you misunderstand what I am trying to say. Let me clarify.

HCA and GIA are in methodology similar systems, working with rather broad average parameters. The result is like fishing with a fine-mazed net, where the fine maze makes you catch 90% of a school of fish. Still, I am sure that you would not value every fish the same.

Even if we consider the strictest lab-system of this moment, that of AGS, their cut-grade does not contain a metric for scintillation yet. It is inevitable that someday there will be refinements making future cut-grades stricter.

This frustrates me mostly on the worldwide wholesale-level. Let me explain. For a cutter, sourcing is considered stategically more important than sales, so my frustration is in sourcing.

Recently, with rough diamonds being extremely expensive, part of our sourcing is concentrated on buying polished diamonds from the production of other cutters, in order to re-cut them to our quality. It is frustrating to see how they value all GIA-EX the same, and if they have something smelling like H&A, the stone will be more expensive. On average, I must reject 90% of what we see, simply because those EX-stones are simply overpriced for me. It is an indication however that the market is accepting and absorbing these prices, mostly based on the broad GIA-grade only.

If I take this exercise to check what is talked about on PS, I regularly see stones for which I would gladly pay 80% of the PS-retail-price, which indicates that these are definitely worth their money. But in the same thread, other stones can be recommended that on paper are similar, for which I would pay only 50% of the PS-retail-price.

Live long,

 

[dicxz80]

I see that GIA has the following parameters for Excellent Cut.


GIA (Excellent) Crown Angle: 31.5° - 36° Pavilion angle: 40.6° - 41.8° Table Size: 52 - 62%

I have located a stone that has the following:

Crown Angle; 33.5
Pavilion: 40.6
Table 59
Depth 59.7

The stone cut is rated by GIA as Very Good.

When I input those into the HCA Advisor they come up with
Excellent, Excellent, Excellent, Excellent. 1.1

It appears as though all of the parameters meet GIA standards. Why the rating particularly when HCA shows it to be excellent throughout? Am I missing something?

based on my noob opinion, could it be because of wider (than usual) table and shallower (than usual) depth ?