The cut of a diamond has the biggest impact on a diamond’s overall appearance, because it affects how well a diamond captures and reflects light back to the viewer's eye. In other words, cut is what makes a diamond sparkle.
The cut grade depends on the quality of a diamond’s proportions and symmetry. When a rough diamond reaches the hands of a diamond cutter, they create the shape and cut of the diamonds. A well-cut diamond will appear brilliant and sparkly, while a poorly cut diamond may appear dark and dull.
The pattern of surfaces (or facets) you see on a diamond represents the quality of the cut and has a direct effect on the overall sparkle of the stone. When a diamond well-proportioned cuts, light reflects from one facet to another, then reflects back through the top of the diamond.