The brilliance and fire of a diamond is largely determined by the cut of the diamond. The proportions that determine the optimum scintillation vary with each diamond shape. Finish refers mainly to the symmetry of the cut. The cut of the diamond is the primary reason a diamond sparkles the way is does. That sparkle and flash of light is what it is because of the play of light within the different facets that are found within the diamond.
The "ideal" proportions (depth and width)of diamonds varies with the different shape of the diamonds.