Regarding the concept of "Ideal Cut", I refer to the President of G.I.A. :

"Developing a fair, workable, and comprehensive way of assessing cut in diamonds is one of the highest priorities underway at the institute. It has been for several years. But because G.I.A. services the world diamond community, such a system must take into account the diversity of regional tastes and preferences based upon visual appearance, the economics of diamond manufacturing, the science of diamond cutting, and ultimately, what is best for the consumers. Above all, G.I.A. cannot elevate one cut standard over another unless there are solid scientific reasons for doing so.

In our view, questions about assessing cut in diamond will be decided in large part by scientific analysis: extensive and expensive research G.I.A. began five years ago, which is now producing results.
So far, our research into the impact of proportions on a diamond's brilliance and dispersion supports a conclusion that we had drawn empirically from our vast experience grading and analyzing millions of diamonds over the past five decades: models such as the so-called "ideal cut" venerated in America and Japan must share the spotlight with other paradigms. In fact, our study of proportions has led us to conclude that the success of cutting has more to do with the interrelationship of three critical measurements - table size, crown angle, and pavilion angle - than a selection of long revered and isolated proportion measurements alone.

Our research has shown that there are thousands of combinations that yield a beautiful round brilliant cut diamond, and an almost infinite number of variations that exist for attractive fancy shapes."

The Loupe
G.I.A. World News
Volume 7 Number 2
Spring 1998