There is cut. And then there's cut.     

           Many people confuse cut with the shape of a diamond. Diamonds are cut into a number of shapes, depending on the nature of the rough stone. The most popular are round, marquise, oval, pear, heart, emerald, princess and trilliant, and the choice is largely a matter of personal preference.       

          Whatever the shape, however, a well-cut diamond is the work of a master diamond cutter. When cut to good proportions, the the diamond is better able to handle light, creating more scintillation, more sparkle. And that's a great reflection on you. 

     It is the cut that enables a diamond to make the best use of light. When a diamond is cut to good proportions, light is reflected from one facet to another and then dispersed through the top of the stone. If the cut of the diamond is too deep, some light escapes through the opposite side of the "pavilion". If the cut is too shallow, light escapes through the pavilion before it can be reflected.    

     The different facets of a round brilliant diamond.

    The eight most popular shapes of a diamond.