The cut and polish of a diamond is the most important of the 4c's. When a diamond is cut with precision and a focus on correct facet placement, it will maximize the diamonds brilliance, scintillation and fire.

When a diamond is cut to ideal proportions, light will enter in throught the top of the diamond, reflect from one facet to another and will then exit towards your eye giving you most brilliance, scintillation and fire.

(PIC 1) 

If the diamond is cut to deep, then some light will escape through the opposite side of the pavilion, or bottom, of the diamond. This will make the diamond look smaller and less brilliant.

(PIC 2)

If the cut is too shallow, then some light will escape through the pavilion, or bottom, before it can be reflected. Diamonds cut this way look steely and flat, having very little life to their sparkle.

(PIC 3)

So a well cut diamond comprised of facets with the correct angles, being perfectly placed will  maximize the diamonds brilliance. It is the quality of the cut and its final polish that unlocks the hidden beauty.