It is important to get a diamond with a good cut.  We must not get confused between a diamonds cut and shape.  The cut of the diamond determines how the light is handled.  

A diamond cut with good proportions will allow light to reflect off one facet to another and then disperse through the top. 

If the cut is too deep, light will reflect off one facet and escape through the other facet. 

If the cut is too shallow, the light will escape through the facet before it can be reflected.