A diamond’s cut determines its sparkle and is therefore crucial to the stone’s final beauty and value. Diamonds are renowned for their high refractive index, i.e. their ability to transmit light and sparkle so intensely. 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. The finer the cut quality, the more sparkle the diamond has.
We often think of a diamond’s cut as shape (round, heart, oval, marquise, pear, etc), but a diamond’s cut grade is really about how well a diamond’s facets interact with light. Of all the diamond 4Cs, the cut is the most complex and technically difficult to analyse.
To determine the cut grade of the standard round brilliant diamond – the shape that dominates the majority of diamond jewellery – 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 colours 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 colour range contains 5 grades: excellent, very good, good, fair and poor.