A diamond cut to optimal proportions, with optimal polish and symmetry, and the most weight loss to produce maximum luster, brilliance, dispersion, and scintillation. Diamonds cut to this standard are the most valuable, with only 5% of the round brilliant diamonds on the market cut to this standard.
An ideal cut diamond has the best combination of angles that allow light to enter the diamond and be reflected back, to create the best combination and brilliance and fire. Ideal cuts often create perfectly aligned arrows and heart patterns that can be seen under magnification.
A diamond cut to utmost precision to maximize brilliance and fire. In 1919, Marcel Tolkowsky calculated a set of measurements that would produce a Round Brilliant cut diamond with the best balance of Brilliance, Dispersion and Scintillation. The AGS now defines an Ideal Cut Diamond, as any diamond which has cut proportions close to that of the Tolkowsky's Ideal Cut.