A diamond's beauty and brilliance is determined primarily by its cut. The differences between various cuts can be quite dramatic, and no cut is more renowned than an ideal cut. An ideal cut is a round-cut diamond that is cut in a predetermined way to give the diamond maximum shine, fire and brilliance. They are cut precisely to mathematically ideal proportions to best highlight the beauty and brilliance of the stone.
Learn how diamond dimensions are defined--by table, crown angle and pavilion. The table is the top, flat surface of the diamond. The crown angles are the side angles that slope down from the table. The pavilions are the longer, angled sides that lead to the bottom of the diamond, also known as the culet. When a diamond is set in a ring, the table and crown angles are the most visible areas, and the pavilion areas are fully set within the ring.
Memorize ideal cut diamond dimensions. As defined by the American Gem Society (AGS), an ideal cut diamond requires a table diameter of 52.4 to 57.5 percent, crown angles of 33.7 to 35.8 degrees and a pavilion depth of 42.2 to 43.8 percent. The total depth of an ideal cut diamond is 58.7 to 62.3 percent.
Know that the AGS also requires that an ideal cut diamond has a girdle thickness that is either thin, medium or slightly thick, and a culet size that is determined as either none, pointed, very small, small or medium. The girdle is the middle or widest part of the diamond, and the culet occurs where the pavilion meets at a point.
Consider additional elements that factor into an ideal cut diamond. While the proportions are essential, ideal cut diamonds should also have very good to excellent symmetry and polish.
Look closely at the diamond in question. It can be difficult to determine the exact proportions with the naked eye. The easiest way to tell an excellent cut from a poor cut is to observe the brilliance of the diamond. If a diamond appears dull, it is likely a very poor cut.