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, the cut makes a diamond sparkle. It is arguably the most important aspect to consider when diamond buying.
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 diamond. A well-cut diamond will appear brilliant and sparkly, while a poorly cut diamond may appear dark and dull.
The pattern of cut surfaces — or facets — represents the quality of the cut and has a direct effect on the overall brilliance. With well-proportioned cuts, light reflects from one facet to another, then back through the top of the diamond.