The cut of a diamond not only refers to the diamond's shape. it also refers to how effectively the diamond returns light back to the viewer's eye. A well-cut diamond will appear very brilliant and fiery, while a poorly cut diamond can appear dark and lifeless, regardless of its color or clarity. Not only do well-cut diamonds appear more brilliant, they also tend to appear larger than other diamonds of the same carat weight. An "ideal" diamond has both increased brilliance and diameter relative to more deeply-cut diamonds.
UNDERSTANDING BRILLIANCE, DISPERSION & SCINTILLATION
A well-cut diamonds exhibit three different properties: brilliance, dispersion and scintillation. As light strikes a diamond's surface, it will either reflect off the table of a polished stone or enter the diamond. The light that is reflected off the diamond is known as the diamond's brilliance. As light travels through a stone, some of the light rays are separated into flashes of color. This is known as dispersion. The result of dispersion—the separation of white light into its spectral colors— is known as fire. Scintillation is flashes of color that are viewable as an observer moves a diamond back and forth.