Which cut score is more reliable?

[sfu88]

Hi Scopers,

I am a newbie and wondering if any of you can help me with my question. I have come across a diamond with GIA cut graded excellent (new cert). When I run the parameters against the HCA, the result was dramatically different (HCA of 4.6). I understand HCA is not a definitive tool for Cut grading but I would imagine the result would be fairly similar to what GIA would have said; however, in this case it is not. Understanding GIA has spent so much resource on cut research I tended to have a lot faith in their new cut grading. However, whenever there is contradiction you''ve got to doubt if your original thought was correct or not. Can anyone give me some advice? Is this a good stone? My sense is that with a total depth of 62.1% it would probably should be considered as too deeply cut.. But the GIA cut grade "excellent" is putting me into puzzle and confusion....

I think this issue must have arised numerous times since the introduction of the GIA cut grade...

Carat : 1.58 D VS1
Table : 55%
Crown Angle : 34.5%
Pav Angle : 41.4%
Star% : 50%
Lower half % : 80%
Girdle min : Thin
Girdle max : Med
Cutlet : None
Polish/ Symmetry : Ex/Ex
Total Depth : 62.1%

Thanks for your comment.

Stephen Fu

 

[esguy27]

This may be a dumb question, but what type of stone is this? I think the HCA only works for round diamonds.

 

[Paul-Antwerp]

It is exactly the kind of GIA-Excellent, that I warned for months ago. I personally foresee that you will see many like these being cut. The new GIA-system is extremely forgiving towards one side, where cutters can maintain the highest weight.

A simple check on the HCA shows that this stone''s light performance is far from excellent, and no extra measurements of lower girdle facets or star sizes can change that.

If you want confirmation that HCA is more reliable here, check out the AGS-cutting charts. This stone would get AGS-3 at best.

Live long,

 

[valeria101]

Date: 2/2/2006 12:28:31 PM
Author:sfu88


Understanding GIA has spent so much resource on cut research I tended to have a lot faith in their new cut grading. However, whenever there is contradiction you''ve got to doubt if your original thought was correct or not.

True enough. You are definitely not the first to get to this point

It may be that GIA spent a fortune on their research, but how much of the actual result made it into the actual grades though the ''filter'' of their commercial commitment... is yet another story.

No less resources went behind the HCA (look up ''The Cut Group'' on this forum) and this is definitely not the only product of that said body of knowledge. Probably ''tip of the iceberg'' is more like it.

The best thing, IMO is to test the stone itself - because both systems HCA and GIA cut grades are approximative (only difference: the HCA recognize this openly...). GIA doesn''t have a purpose designed tool to check their grading performance diamond by diamond in person. The HCA has - the IdealScope.

When technology gets over my head (easy that!), I would still choose the more transparent system. And in this case it is the HCA. Much of the research behind it in the open for anyone to take a dive into. GIA''s finds are proprietary - if you trust them, you must trust a black box, like it or not...

My 2c

 

[sfu88]

Thanks folks!

I was moving in the last few days and wasn''t able to check the forum. Thanks alot for all of your advice.

Stephen

 

[Regular Guy]

Stephen,

This question is a rather hot point of discussion on this board, with the focus less being on whether GIA''s cut grades are problematic or not, but rather, the focus seems to be on understanding just how bad they are. Personally, I had been a fan of their efforts until a couple of months ago, when the topic addressed in the "pinned" thread at the top of this board right now by JohnQuixote came to light, and where I thought what they were doing, in a worst case, could at least allow users to take advantage of systems like the HCA by finally publishing crown & pavillion angles...turns out their rounding disables a trustworththy use of even this presentation of data, regardless of the additional analysis they bring to the picture.

Do see the thread at the top of the page, check out Garry''s second post on page one, and the chart attached as a file for an interesting reference. There''s generally lots of diamonds to choose among, and unless you''ve figured this all out, if you really want to bank on GIA, consider looking for an "intersecting set of angles," so that you''re taking advantage of a match between systems. Like you''re doing.