The cut of a diamond, the roundness, depth, width and facets all determine the diamond's brilliance. Many feel the most important characteristic of any diamond is the cut. A diamond with perfect, clarity, color and carat weight is uneventful in the face of a poorly cut stone.
How light travels through the stone determines how well the diamond was cut. If the stone was cut too shallow, light is lost out of the sides causing the diamond to lose brilliance. If it is cut too deep, light escapes out of the bottom causing the diamond to appear dark and dull.
Learn more about diamonds and GIA Diamond Grading Reports