Shape and cut difference

Don't confuse diamond "cut" with "shape." Shape refers to the general outward appearance of the diamond, (such as round, emerald, pear). “Cut” is a reference to the diamond's reflective qualities, not the shape. The better a diamond is cut, the more light it will reflect with the result that the diamond has more brilliance. This means that there are good and bad cuts in every shape.
 

As shown in the images below, when a diamond is well-cut, light enters through the table and travels to the pavilion where it reflects from one side to the other before reflecting back out of the diamond through the table and to the observer's eye. This light is the brilliance we mentioned, and it's this flashing, fiery effect that makes diamonds so mesmerizing.



 

In a poorly cut diamond, the light that enters through the table reaches the facets and then 'leaks' out from the sides or bottom of the diamond rather than reflecting back to the eye. Less light reflected back to the eye means less brilliance.