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Calling Garry H (Cut Nut) - How does the HCA score work?

UnionGuy

Rough_Rock
Joined
Oct 18, 2016
Messages
1
Hi Garry!

Ive been using the PriceScope diamond search tool and see the diamonds are listed with your HCA score. I cannot for the life of me understand how the #'s are derived. Could you explain how it works and why it works?

If the score is lower does it mean it will be a better diamond than one with a higher score?

How did you come up with the system. Does it work for all round diamonds?

How accurate is it? A diamond expert told me it has some preferential treatment towards something called steep and deeps?
 

Karl_K

Ideal_Rock
Trade
Joined
Aug 4, 2008
Messages
9,466
UnionGuy|1476838891|4088425 said:
Hi Garry!

Ive been using the PriceScope diamond search tool and see the diamonds are listed with your HCA score. I cannot for the life of me understand how the #'s are derived. Could you explain how it works and why it works? I am going to let Garry tackle that one

If the score is lower does it mean it will be a better diamond than one with a higher score?under 2 is considered good. But it must be keep in mind it is not as ring based as other systems and gives a good score to some shallow pavilion stones that are potentially great in earrings and pendants but not optimal for rings.

How did you come up with the system.

Does it work for all round diamonds?modern round brilliant cuts only and makes some assumptions like a med girdle and does not take into account the minor facets and uses averaged numbers and in the case of GIA report numbers they are very grossly rounded.

How accurate is it? useful as a sorting tool to find some stones to gather further information, such as reflector images.

A diamond expert told me it has some preferential treatment towards something called steep and deeps?
It is in some cases overly harsh on moderate steep deep combinations and for ring use overly generous on shallow pavilion combinations. It is the GIA system that gives its excellent score to many overly steep deep combinations.
 

carbonquest

Rough_Rock
Joined
Nov 4, 2016
Messages
26
steep and deep? I thought it favoured shallower crowns? then again I may be mistaken.

As i understand it - the lower the HCA score the better. people usually prefer 1-2. Scores <2 can be considered, and scores over 2 can be discarded.

It is an AID and NOT A SELECTION tool for stones. For example if you had your heart torn between two GIA triple X's, and one scored 2.2 and one scored 1.1 - then you know which one is more 'likely' to be superior? Im sure there is more information on this site somehere about the numbers etc.
 

gm89uk

Brilliant_Rock
Premium
Joined
May 26, 2015
Messages
1,449
I think it is a simplified form of ray tracing based on a generalised algorithm that detects hypothetical light leakage of a diamond. Assumptions are that the diamond is perfectly symmetrical and has predefined LGF and star %.

Regarding fire and scintillation I have been perplexed by some of the outcomes, such as

36/40.6 with a 54% table and 62% depth would have very good scintillation rather than excellent, but a 32/40.8 with 58% table and 60% depth would have excellent scintillation.

This goes against my understanding that scintillation that comes primarily from the crown.
 

Texas Leaguer

Ideal_Rock
Trade
Joined
Jul 27, 2009
Messages
3,190
carbonquest|1480168658|4102723 said:
steep and deep? I thought it favoured shallower crowns? then again I may be mistaken.

As i understand it - the lower the HCA score the better. people usually prefer 1-2. Scores <2 can be considered, and scores over 2 can be discarded.

It is an AID and NOT A SELECTION tool for stones. For example if you had your heart torn between two GIA triple X's, and one scored 2.2 and one scored 1.1 - then you know which one is more 'likely' to be superior? Im sure there is more information on this site somehere about the numbers etc.
Carbon,
You can find more information within the pages of the HCA tool, both on the input screen and on the results screen. There is even a link to the original patent.

It is important to recognize the limitations of the tool and not try to do too much with it. Its primary value is as a proportions checker that will identify diamonds with problematic combinations. There is a natural tendency to want a simple, quick, and thorough assessment. HCA certainly does the first two! But in the author's own words, an ideal scope image of the diamond is more useful than HCA.

Before and for a period after development of HCA , ideal scopes were not widely available. Today many merchants provide ideal scope, ASET, and other diagnostics such as H&A showing optical precision. Furthermore, AGSL released their light performance grading system a few years after HCA, which ray traces an accurate model of the actual diamond, accounting for the contribution to light performance of every facet. HCA is a table-based tool similar to the online GIA facetware tool. The results are driven by comparing the inputs to pre-defined grades on tables that were developed by the respective authors. While Facetware takes more data into account, neither looks at the actual diamond.

Trying to make fine distinctions about performance of two stones on the basis of an HCA score is futile because it only looks at basic averaged measurements. Using inputs from a GIA report further undermines accuracy of HCA scores because of rounding.

The main value of HCA is to sift through large numbers of online offerings to eliminate those that are out of bounds from a proportions standpoint, allowing you to focus on acquiring more information on a select group of candidates. The pricescope diamond search makes this very handy by automatically incorporating HCA scores in the search results.
 

Texas Leaguer

Ideal_Rock
Trade
Joined
Jul 27, 2009
Messages
3,190
gm89uk|1480171032|4102728 said:
I think it is a simplified form of ray tracing based on a generalised algorithm that detects hypothetical light leakage of a diamond. Assumptions are that the diamond is perfectly symmetrical and has predefined LGF and star %.

Regarding fire and scintillation I have been perplexed by some of the outcomes, such as

36/40.6 with a 54% table and 62% depth would have very good scintillation rather than excellent, but a 32/40.8 with 58% table and 60% depth would have excellent scintillation.

This goes against my understanding that scintillation that comes primarily from the crown.
GM,
Scintillation is the least understood of the light performance factors that determine diamond beauty. If you look at the HCA patent you will see a statement to this effect as well. It does assess head shadow which is structured contrast that contributes to scintillation, but it does not give you a full picture of scintillation by any means. Even the AGSL light performance grading, while ray tracing the actual diamond and calculating values for light return, light leakage, fire, and contrast, does not specifically grade scintillation. Advanced research in this area is going on and I think you will see that capability developed in the future.

It would come in very handy in understanding the "flavor" of the diamond. Especially in regards to fancy shapes which can range widely in visual appearance and appeal, for example crushed ice vs brilliant styles. But also with respect to rounds of different proportion combinations or faceting configurations.
 
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