A diamond cut is a style or design guide used when shaping a diamond for polishing such as the Brilliant Cut. Cut does not refer to shape (pear, oval), but the symmetry, proportioning and polish of a diamond. The cut of diamond greatly impacts diamond brilliance, this means if it is cut poorly, it will be less luminous.
The choice of diamond cut is often decided by the original shape of the rough stone, location of internal flaws or inclusions, the preservation of carat weight, and popularity of certain shapes amongst consumers. The cutter mush consider each of these variables before proceeding. Polish and symmetry are two important aspects of the cut. The polish describes the smoothness of the diamond's facets, and the symmetry refers to alignment of the facets. With poor polish, the surface of a facet can be dulled, and may create blurred or dulled sparkle. It may constantly look like it needs to be cleaned. With poor symmetry, light can be misdirected as it enters and exits the diamond.
The round brilliant diamond consists of 58 facets (or 57 if the culet is excluded); 33 on the crown (the top half above the middle or girdle of the stone) and 25 on the pavilion (the lower half below the girdle). The girdle may be frosted, polished smooth, or faceted. In recent decades, most girdles are faceted; many have 32, 64, 80, or 96 facets; these facets are excluded from the total facet count. Likewise, some diamonds may have small extra facets on the crown or pavilion that were created to remove surface imperfections during the diamond cutting process. Depending on their size and location, they may hurt the symmetry of the cut and are therefore considered during cut grading.
Hearts and Arrows
A diamond that has the top facet or "table facet" exactly perpendicular to the bottom of the diamond or "pavilion" and has its other facets precisely aligned with excellent symmetry, may show patterns that look like arrows from the bottom and hearts from the top. Generally it will need to be viewed loose under a gem scope to see the pattern very well. Although the hearts and arrows property is indicative of a top-tier cut, it does not always mean the diamond will be the most brilliant. Optimal facet placemen is the key to brilliance and more important than facet patterning. Not all ideal round cuts will have the hearts and arrows effect either.
When a diamond has a high quality cut (ideal cut), incident light will enter the stone through the table and crown, traveling toward the pavilion where it reflects from one side to the other before bouncing back out of the diamond's table toward the observer's eye. This phenomenon is referred to as "light return" which affects a diamond's brightness, brilliance, and dispersion. Any light-leakage caused by poor symmetry and/or cut proportions will adversely affect the quality of light return.
Excellet Represents roughly the top 3% of diamond quality based on cut. Reflects nearly all light that enters the diamond, an exquisite and rare cut
Very Good Represents roughly the top 15% of diamond quality based on cut. Reflects nearly as much light as the excellent cut, but for a lower price.
Good Represents roughly the top 25% of diamond quality based on cut. Reflects most light that enters. Much less expensive than a very good cut.
Fair Represents roughly the top 25% of diamond quality based on cut. Still a quality diamond, but a fair cut will not be as brilliant as a good cut.
Poor Diamond that are generally so deep and narrow or shallow and wide That they lost most of the light out the sides and bottom.
You may check your diamond's cut grade using GIA's facetware https://www.gia.edu/facetware/