A diamonds cut is undeniably the most important of the 4 Cs. Even with close to perfect colour, size and clarity if a diamond’s cut is not in correct proportions all other characteristics will not be as highly valued. The way in which a diamond is cut will determine how the diamond reflects light and hence determine its brilliance or sparkle. A poorly cut diamond will appear lifeless and dull.
Diamonds reflect their light from one facet to the other. If the cut of the diamond is too deep then light will escape through the opposite side of the pavilion. On the other hand if the cut is too shallow then light will escape through the pavilion before it can be reflected.