GIA "Excellent" vs. AGS "Ideal"

[Nemesis]

Hello,

I''ve been lurking here for the past couple of weeks or so. This is time to step out of the shadows and post.

I was at a local jeweler yesterday who seemed to be alright (i.e. not pushy, took the time to explain about diamonds, why cut is the most important factor, etc.). He indicated that between the GIA and the AGS, that a GIA "excellent" rating was probably at the upper end of the AGS "ideal" rating. Meaning that GIA excellent ratings always warrant a AGS ideal rating in terms of polish and symmetry, but that an AGS ideal could either end up as a GIA excellent or a GIA very good rating, depending on the stone. This is the first time that I''ve heard or read about this anyplace so was wondering if anyone more experienced has any comments about this.

Also, he mentioned the height of a girdle as a percentage and why girdle height was important. From my reading, I knew that I should be looking for thin/medium/slightly thick, but I never saw it displayed as a percentage. He was referring to some pamphlet he had in his store and said that ideal stones had girdles less than 2.2%? I''m guessing that even slightly thick girdles will fall beneath that 2.2 range?

That''s about it. Curious to see what you all think about his comments.

Thanks for all the information and to PS for hosting this site. There is such a good wealth of good information here!

 

[Cath]

Nemesis, welcome to Pricescope. I can''t answer your question about GIA vs. AGS, but here''s some information Leonid once posted about girdle measurements. Based on the below, a girdle thickness of 2.2% is within the "slightly thick" definition.





Different labs use different systems for girdle thickness: GIA measures mm while AGS %.
GIA
Extremely thin 0.00 - 0.08 mm
Very thin 0.08 - 0.12 mm
Thin 0.12 - 0.15 mm
Medium 0.15 - 0.20 mm
Slightly thick 0.20 - 0.23 mm
Thick 0.23 - 0.33 mm
Very thick 0.33 - 0.40 mm
Extremely Thick: greater than 0.40 millimeters
AGS
Very thin 0.091 - 0.50%
Thin 0.51 - 1.2%
Medium 1.21 - 1.7%
Slightly thick 1.71 - 2.95%
Thick 2.96 - 4.2%
Very thick 4.21 - 5.7%
Extremely Thick: >= 5.71 %

 

[Lord Summerisle]

Hmm.. kinda tricky one to decipher, if I read it correctly.

AGS grade diamonds on its proportions (ie cut) its colour, clarity, polish, symmetry and remark on its weight, its proportions (depth % table% crown angle and % pavilion angle and % etc.) and its inclusions.

GIA does the same, except it does not grade its proportions, but simply state its depth and table size.

So… while a diamond sent to AGS could be rated Good for its cut, since GIA does not take this into account they wont grade.

An AGS Ideal – could be seen as reference to the cut grade (plus poss its polish and meet point symmetry), while a GIA excellent grading is referring to the meet point symmetry and its polished finish only.

A report on here concluded I believe that AGS where often a little stricter than GIA (which is seen as a benchmark for lab grading) in terms of their gradings. And as such would present a mirror of what u say about the grade results. Ie AGS being the stricter lab ment a GIA excellent could result in either a AGS v good or ideal grading. But for the most part both labs where the same.

But also AGS could grade a diamond Ideal cut (0) but good for its polish… so it could be sold as a AGS Ideal… while GIA would grade the diamond a ‘Good’ purely as cut isn’t factored in..

This is all from what I remember of a long report I read on here, so I may be wrong on this one*

Any help?

*standard issue Summerisle Disclaimer

 

[pqcollectibles]

Ex/Ex is GIA''s top rating while Id/Id is AGS''s top rating. For screening purposes, you can consider AGS Id/Id and GIA Ex/Ex to be pretty much equal. Then you will need to get more in depth info,.......... Sarin, Ideal Scope, etc, for true comparison purposes when considering specific candidates.

 

[Nemesis]

Thanks for the replies. For some reason, I had it in my head that the AGS was the more strict of the two (if just slightly so) so it seemed odd what he said last night. I wasn''t sure if something changed in standards between when I looked for diamonds a few years ago and now.

Thanks!

 

[pqcollectibles]

In the top toolbar, highlight Reviews, then click on Grading Survey and you can read an article about a comparison of the various grading labs.