Cut Rating - GIA Excellent vs. AGS Ideal

First and foremost, I apologize if this issue has been done to death. I ran a search and read through several threads about AGS vs. GIA cut grades, but I haven''t really found the information I''m looking for. I understand the general agreement here and at other internet resources is that an AGS Ideal cut grade generally represents stricter requirements for cut quality than GIA Excellent. For example, most diamonds that rate AGS Ideal would also qualify as GIA Excellent, but not necessarily vice-versa.

I''m currently shopping for an engagement ring, and in addition to searching online I''ve been speaking to several local jewelers to ensure I do my due diligence before making such an expensive purchase. The reason I''m posting is due to a conversation I had with one such jeweler.

He was of the opinion that I was overly narrowing my search criteria by only considering stones with an AGS 000 certification and avoiding GIA certified stones. I explained to him that my research indicated that AGS had a narrower band for considering a stone an ideal grade than the GIA did for an excellent. He pressed me on this a bit, stating that he felt that GIA and AGS were similar as far as cut rating, suggesting that, if anything, he felt GIA was a superior certifying body because of the volume of stones they graded. I should note that this gentleman was a certified gemologist with a history at a grading lab, not just a diamontologist trying to sell me a ring (the sales rep went and got him when I started inquiring about the differences between the labs and cut quality). It''s true that most of his "in-house" diamonds were GIA certs, but since he could always order AGS stones I''m willing to give him the benefit of the doubt for the time being that he wasn''t trying to pull some sales trick on me.

From what I understand, the main difference is that AGS provides a grade based on the proportions of the diamond, in addition to the polish and symmetry, while GIA only provides polish/symmetry. Is this correct? If so, can I make a cut determination for a GIA certified stone just by knowing its measurements?

If I''m interpreting the difference wrong, could someone provide me with a breakdown of the debate of the different cut grading philosophies the GIA and AGS use, and why AGS generally has stricter criteria for an ideal cut? I''ve seen several articles on cut grading, but most seem to state "AGS is stricter" as a matter of fact without all the facts to support such a conclusion. I''d really appreciate a lay-man''s terms breakdown of why AGS has a superior cut grading methodology to GIA.

Finally, is he correct that I am limiting my selection by only considering AGS stones, and I should be willing to consider well-cut GIA stones as well? Can I determine if the cut quality of a GIA certified stone would qualify as AGS ideal just from the measurements?