The Cut of a diamond which is not to be confused with the shape of a diamond determines its brilliance.
Most gemologists consider cut the most important diamond characteristic because even if a diamond has a perfect colour and clarity a diamond with a poor cut will have dulled brilliance.
When a diamond has been cut to shallow the light is lost out the bottom causing the diamond to lose brilliance and if a diamond has been cut to deep light escapes out the sides causing the diamond to appear dark and dull.
There is no single measurement of a diamond that defines its cut, but rather a collection of measurements and observations that determine the relationship between a diamonds light performance, dimensions and finish.
High quality cuts are produced to maximize the light performance and brilliance, taking far more time to produce and requiring greater skills of the cutter, therefore the better the cut the better the brilliance and the higher the finished cost and therefore the higher the value.