Because diamonds are so valuable, it’s essential for industry professionals to have a universal grading system when comparing diamond quality. In the mid-twentieth century, GIA developed the International Diamond Grading System™ and the 4Cs as a way to objectively compare and evaluate diamonds.
An ideal cut diamond will reflect light from the many combinations of crown and pavilion angels to show it's brightness and fire. Cuts too shallow lose light out the bottom causing the diamond to lose brillance. Cuts too deep causes light to escape out the sides making the diamond appear dull and dark.