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Yet another HCA thread....GIA Ex and Vg vs. HCA

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kristie

kristie

Shiny_Rock
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Mar 18, 2009
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While I was carousing diamonds online I was amazed that I found numerous GIA triple Ex. stones that scored poorly (4.9) on the HCA.

Meanwhile, I found a gorgeous GIA Vg/Vg/Vg that scored a 1.4 on the HCA

Which stones will look "better"? The GIA triple Ex. that is a poor performer on the HCA or the Vg that scored a 1.4?

I am assuming the HCA is a more reliable indicator of the actual performance than just GIA cut parameters.
 
The new stone got a VG in cut grade because of the the V-Thin in the girdle variation and a 85% lower half. GIA downgrades these 2 VG cut grades.

EDT:
Check facetware, it was downgrade because of the V-Thin girdle, 85% lower half is still acceptable for this combination.
 
Date: 2/10/2010 12:42:21 AM
Author:kristie
While I was carousing diamonds online I was amazed that I found numerous GIA triple Ex. stones that scored poorly (4.9) on the HCA.

Meanwhile, I found a gorgeous GIA Vg/Vg/Vg that scored a 1.4 on the HCA

Which stones will look 'better'? The GIA triple Ex. that is a poor performer on the HCA or the Vg that scored a 1.4?

I am assuming the HCA is a more reliable indicator of the actual performance than just GIA cut parameters.
Click to expand...
Hi Kristie

Do you have a link to the diamond that is a VG cut grade? The HCA is a rejection tool, not for selection but from your scores one could assume that the VG cut grade could look better than the probable steep deep angled GIA Excellent. Various factors which aren't detrimental to the beauty of the stone can cause a VG cut grade such as a very thin girdle being a common one - not always the case that a very thin girdle will be a durability issue but each need evaluating on a case by case basis.

As to GIA Excellent and low HCA scores, this has been a factor since GIA have been offering cut grades, as the GIA Excellent grade allows for steep deep angled stones to get the Excellent grade, these diamonds can show light leakage which is undesirable.

Here are some old threads from early days of the cut grading you might find interesting;

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Essentially, the HCA and the GIA cut-grade are very similar in set-up. They take a limited number of average measurements of the stone, and use that as a predictor for potential cut-quality. This obviously leads to the same limitations, in which it is difficult to say that one tool is better than the other.

The small differences in outcome however are interesting.

Indeed, it happens regularly that GIA-EX-stones score low on HCA. One of the factors in this is that a strict division in 5 grades (GIA: EX, VG, G, ...) is less refined than a number ranging from 0 through 10. Most important however is that GIA is more lenient in what we call the steep/deep-area, where HCA correctly penalizes these stones.

On the other hand, the GIA using a few more measurements can also be a benefit, since in this case for instance, it picks up the very thin girdle (durability risk) and the long lower halves.

As for your questions, it is impossible to state on this basis only which stones will look better. I am inclined to say that in your examples, the HCA correctly downgraded the GIA-EX-stones, but also that GIA correctly downgraded the 1.4-HCA. As such, neither tool is a more reliable indicator of the actual performance, and using both as rejection-tools, none of these stones deserve further consideration.

Live long,
 
^^^^Neither of them deserves further inspection?? I saw the 2.68 (The GIA Vg/Vg/Vg) that the link was posted to IRL and it was gorgeous....*now more confused than ever*
 
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