The cut of a diamond refers not to its shape, but to the balance of proportion, symmetry, and polish achieved by the diamond cutter. The extent of how well the diamond is cut is directly related to the diamond's overall beauty. When a diamond has been correctly cut, the diamond's ability to reflect and refract light is greatly enhanced. By understanding the way that light moves through diamond crystals, modern diamond cutters have established a specific set of proportions and angles that are known to harness the diamond's internal brilliance and to show it in its best light. A diamond's cut grade is actually a combination of three different types of reflection:
Brilliance, or brightness, refers to the white light that is reflected back to the eye from the diamond. Light enters through the top of the diamond (the table), is broken down into a rainbow of spectral colors and is reflected back and forth in the interior of the gem by bouncing off the mirror-like facets. Light exits through the table, recombining as white light.
Dispersion is the rainbow of colors that is reflected back to the eye from the diamond. Light enters through the top of the diamond, is broken down into a rainbow of spectral colors and is reflected back and forth in the interior of the gem by bouncing off the mirror-like facets. When it leaves through the crown, it stays separated and reaches the eye in flashes of color.
Scintillation is the play of light you see with movement of the diamond, demonstrated by sparkling on the diamond's surface. A diamond is evaluated on its ability to reflect and refract light in all directions.
Cut is considered to be the most important of all of the diamond characteristics, as a well-cut diamond will often appear larger than a poorly cut diamond of the same carat weight, and have the appearance of enhanced color and clarity. It is an important factor to consider when choosing a stone for a diamond engagement ring.
The quality of cut is determined by how well the symmetry, polish, and proportions of the diamond produce the most attractive balance of the three different types of reflection. Several proportion factors have the most immediate impact on a diamond's ability to reflect light correctly. The table size and depth of a diamond relative to the diameter greatly impacts the light return from a diamond. A well-cut diamond is proportioned so that most of the light entering the gem exits back through the top of the stone, perfectly balancing the white light (brilliance) with intense flashes of fire (dispersion). A poorly-cut diamond, with facets cut only a few degrees out of alignment, can result in light exiting through the bottom of the diamond, known as light leakage, instead of from the top where it is visible. This creates a diamond with dulled brilliance from poor light performance within the gem, making the center of the gem look dark.
The best cut diamonds have proportions that are within tried and true ranges known for maximizing brilliance, fire, and scintillation. The cut grading scale for diamonds is based on the reflective properties of the diamond, according to these carefully calculated diamond measurements. It also allows you to easily identify a well-cut diamond, without having to assess each individual cut characteristic.