GIA rating for an ideal cut diamond

GIA does not rate cut. They have an EXCELLENT symmetry grade and an EXCELLENT polish grade as their highest marks but do not formally acknowledge CUT grades on their reports like AGS does.

GIA has been at the study of CUT for years and is widely anticipated to add CUT characteristics at some point.

For the straight from the horse's mouth view, I'll cite to 2 articles published in Professional Jeweler (and available in their archives online):

1) "The use of the term 'Ideal' is ... confusing, GIA President William E. Boyajian says.... Although it is not GIA's role to discredit the concept of an 'Ideal' cut, on the basis of our research to date, we cannot recommend its use in modern times." GIA's Brilliant Diamond Cut Study, January 1999

2) "...we are very close to answering the key questions involved in proportion evaluation for round brilliants, GIA President William E. Boyajian writes...." Study Moves GIA Closer to Cut Grade, January 2002.

These citations allude to GIA's long computer modeled study of various combinations of Table, Depth, Crown Angle, Pavilion Angle, Girdle and Culet measurements as they relate to brilliance, fire, and scintillation. GIA to date has refused to put the numbers on their report because they haven't narrowed down the range yet (or so they say; there may be other political factors at work).

AGS has loudly declared the following proportions to be "ideal" (and garnered a lot of business in the process):

Table diameter: 53-57
Crown Angle: 34 degrees to 35.5 degrees
Girdle: Thin; Medium; Slightly Thick
Pavilion Depth%: 42.5-43.5%
Culet: Pointed; Very Small; Small; Medium

The accepted industry standards for Ideal cut fall into this range. Keep in mind that ideal cuts are sometimes expressed also in terms of Pavilion Angle (the ideal is thought to be 40.75 degrees) and Total Depth (the ideal is thought to be between ~59.9% and ~63%.

It's more complicated. The relationship of the crown angles, pavilion angles and depth% and table size vary considerably stone by stone. Symmetry relationships, minor facet lengths/percentages and other mystery factors dictate that even some diamonds which fall within the "ideal" range are not particularly stunning. GIA asserts that even AGS 5 grades can be astoundingly brilliant. See that January 1999 article. Rhino at Good Old Gold has an AGS 5 somewhere on his site which has good light return characteristics.

Look at it this way: what you are ultimately seeking to analyze is the way light bounces around inside of a chunk of clear carbon crystal. From a scientific standpoint, the precision of the angles and their inter-relationships are highly critical in order for the light to bounce out of a diamond in a way that is pleasing to your eye.

Now for the real truth: most diamonds that are cut even marginally well and are of decent clarity are pretty and sparkle nicely. You start paying premiums for them when their cuts get closer to the mythical ideal because truly ideal cut diamonds really do outperform other ones. But, as GIA asserts for the moment, there are a lot of combinations that make for truly brilliant diamonds.