Of the four Cs, cut is perhaps the most important factor affecting a diamond's overall quality and beauty. A diamond's brightness, or its brilliance, is determined by how much light is reflected back to your eyes. Light enters the stone through the crown, which is the portion of the diamond above the girdle. The crown is made up of the table, which is the large flat facet on top of the diamond, and many crown facets. It then travels to the pavilion, or body, of the stone, where it is reflected from one side to the other and then back through the top and to an observer's eye.
A well cut, well proportioned stone evenly reflects and refracts light within the stone, thereby producing an eye-catching, fiery spectrum of color. A poorly cut diamond, on the other hand, allows more light to pass through or leak from the sides of the stone, which results in a lifeless appearance with reduced sparkle.
The science behind diamond cutting is more or less an exercise in proportion. Changing the proportion of a diamond's depth and width is done in order to maximize the stone's brilliance. If the cut adheres to certain ideal proportions, the results can be spectacular. If poorly cut, the results can be so bad as to cause structural instability, which makes the stone susceptible to breaking. Because cut is so important, gemologists have developed grading methods to assist consumers in determining a diamond's cut. In general, they are: Ideal, Excellent, Very Good, Good and Fair.
The ideal proportions shown below are known as the Tolkowsky Theoretical Brilliant Cut. In 1919, Marcel Tolkowsky published a paper detailing ideal proportions for a round brilliant diamond. Modern cutters offer a wide range of opinions on the ideal cut, but Tolkowsky's findings are mathematically indisputable and remain the basis for these modern ideal proportions.