Diamonds are renowned for their ability to transmit light and sparkle so intensely. We often think of a diamond's cut as shape (round, oval, pear), but a diamond's cut grade is really about how well a diamond's facets interact with light.
Precise artistry and workmanship are required to fashion a stone so its proportions, symmetry, and polish deliver the magnificent return of light only possible in a diamond.
A diamond's cut is crucial to the stone's final beauty and value. Of all the diamond 4Cs, it is the most complex and technically difficult to analyze.
To determine the cut grade of the standard round brilliant diamond - the shape that dominates the majority of diamond jewelry - GIA calculates the proportions of those facets that influence the diamond's face-up appearance. These proportions allow GIA to evaluate how successfully a diamond interacts with light to create desirable visual effects such as:
Brightness: Internal and external white light reflected from a diamond
Fire: The scattering of white light into all the colors of the rainbow
Scintillation: The amount of sparkle a diamond produces, and the pattern of light and dark areas caused by reflections within the diamond.
GIA's diamond cut grade also takes into account the design and craftsmanship of the diamond, including its weight relative to its diameter, its girdle thickness (which affects its durability), the symmetry of its facet arrangement, and the quality of polish on those facets.
The GIA Diamond Cut Scale for standard round brilliant diamonds in the D-to-Z diamond color range contains 5 grades ranging from Excellent to Poor.
Source of information: http://4cs.gia.edu/