Making the Grade
Most people call a diamond's shape "cut." Since January 2006, however, the Gemological Institute of America (GIA) started using the term to refer to the quality of a round polished diamond's workmanship, known as Cut Grade. Cut Grade considers the workmanship in transforming a diamond from rough to polished. It is the only characteristic of a diamond determined by human action.
The cut grade deconstructs a round diamond's beauty into many specific and precise components, mainly taking into consideration the proportions of a diamond. It is the symmetry and polish of a diamond, however, and the combination of the quality of the three determines a diamond's Cut Grade. Simply put, the better the diamond's proportions, the more light reflects out of it. In this regard, light is measured by three attributes:
Brilliance – the total light reflected from a diamond.
Fire – the dispersion of light into the colors of the spectrum.
Scintillation – the flashes of light, or sparkle, when a diamond is moved.
For a full read click here