Cut
More than any other attribute, it is the cut that makes a diamond brilliant and beautiful. A diamond that is well cut reflects light internally and displays a dazzling shower of light back through the top of the stone. A poorly cut diamond will leak light out of the bottom and sides, as illustrated below. Symmetry is another important factor in the brilliance and beauty of a diamond. A diamond is like a small house of mirrors. If the facets are lined up properly, the diamond returns more light to the viewer's eye. Finally, a diamond that is well polished will reflect more light and sparkle from the surface to the viewer's eye. Cut, symmetry and polish are rated on your diamond certificate from poor to excellent.
Fast Fact: The measurements listed on your diamond grading report also give an indication of how brilliant your diamond will be. The formula for cutting a diamond of ideal proportions was written in 1919 by Marcel Tolkowsky. Although it is rather complex, you can find the information by searching his name from within any popular internet search site.