The quality of the cut, or the “make,” is crucial. It is this that decides how the light entering the polished diamond through its table – the largest area, facing upward – will react, and consequently how brilliant the stone will appear. A poorly cut diamond will actually loose light and appear dull, and its value will be significantly lower than a diamond with of the same size, color and clarity, but of an excellent cut.
Diamond dealers sometimes use the word “cut” to refer to the shape of the diamond, but in reference to the 4 Cs it specifically refer to the physical factors influencing the interplay of light within and from the polished diamond.
These, according to research carried out by the Gemological Institute of America (GIA) decide the stone’s brightness, which refers to the internal and external white light reflected from the stone; fire, which is the scattering of white light into all the colors of the rainbow; and scintillation, which quite simply is the amount of sparkle a diamond produces, and refers to the pattern of light and dark areas caused by reflections within the gemstone.
To determine the cut grade of a diamond, GIA considers the proportions of those facets that influence the diamond’s table-up appearance. It also takes into account the design and craftsmanship, and the quality of polish on the facets. GIA provides an overall cut grade for standard round brilliant cut diamonds in the D-to-Z color range, with the grade appearing as either: Excellent, Very Good, Good, Fair, or Poor.
Currently GIA does not provide an overall cut grade for non-standard round brilliant diamonds or fancy shaped diamonds. But its reports do contain an assessment of the diamond’s polish and symmetry; a description of its shape and cutting style; its measurements; and a simplified proportion diagram containing a description of girdle thickness.