How a diamond has been cut, polished, and to what proportions and symmetry, are of utmost importance.
These factors determines brilliance and sparkle of the diamond.
If these cutting factors fall below standard, the appearance of the diamond will be adversely affected.

Some of the most common diamond shapes are:
round, emerald, princess, radiant, oval, pear and marquise. Round cut diamonds are the only shape that can achieve an ideal cut, so are the most brilliant of the diamond shapes.
Fancy shaped diamonds adhere to different proportion guidelines and thus refract light in different ways.