Cut
The cut (or proportions) of a diamond is considered the most important characteristic of a diamond in the diamond industry. Cut refers to the angles and proportions a skilled craftsman creates in transforming a rough diamond into a polished diamond. A well-cut diamond will reflect light internally from one mirror-like facet to another, dispersing it through the top of the stone. Cuts that are too deep or too shallow lose or leak light through the side or bottom, affecting the diamond's brilliance.
Diamond cutting requires great skill and training. The cutter must polish tiny surfaces known as facets onto the rough diamond. This process is what creates the facets known as the crown, culet, table, girdle and pavilion of the diamond. The facets, when arranged in precise proportions, will maximize the fire and brilliance of a diamond. To cut a diamond perfectly, a craftsman will often need to cut away more than 50% of the rough diamond.
The proportion between the depth percentage and the table percentage determines a great deal of the brilliance you will receive from your diamond. The depth percentage is determined by dividing the depth in millimeters of your diamond by the width in millimeters. The optimal depth percentage for a round brilliant cut diamond is between 57% and 64%.
From one side of the girdle to the other side of the girdle is 100% of a diamond's diameter. The top flat portion of a diamond, referred to as the table, covers a percentage of that 100%. The table does not cover the entire 100% because the crown angle takes up some of the percentage. The optimal table percentage is between 53% and 64%.