If a diamond is poorly cut, the light that enters the diamond from above will leak out of the sides and bottom of the stone, and the diamond will look lifeless—without sparkle or fire—regardless of its color or clarity.

But an ideal cut is more than a matter of geometry. Thanks to the mathematician Marcel Tolkowsky, we know exactly which angles optimize the diamond's efficiency as a refractive vessel of light.

Yet natural inclusions (or imperfections) in the rough diamond mean that it is not always possible to apply these perfect proportions to every diamond that is cut. For this reason, cuts are graded as follows:

An Ideal cut falls within the strict mathematical proportions illustrated above. Because symmetry is required for maximum light reflection, only round diamonds may be graded as ideal cuts.

A Very Good cut reflects back as much as 90% of all light that enters the stone. Very good cuts are considered exceptionally proportioned, investment quality stones.

A Good cut describes a well-proportioned stone that reflects back a good proportion of light. Good cuts are well suited as jewelry stones.

A Fair cut diamond has been cut to maximize the weight of the original stone, at the expense of fire and brilliance. A fair cut may reflect as little as 40% of the captured light back through the table of the diamond.

A Poor cut diamond appears lifeless to the eye. Poor cut stones are not recommended for fine jewelry, and we do not offer poorly cut diamonds for sale to our customers.

A diamond's cut is graded according to the proportions of the its depth percentage (the height vs. the width of the stone) and table percentage (the diameter of the top facet of the stone vs. the stone's total width). These proportions are detailed on the diamond certificate. When evaluating a diamond's cut, you should also check the diamond's specifications against the following table to determine whether the polish, symmetry, girdle, and culet of the diamond fall within acceptable standards.

Round diamonds, because they are symmetrical and capable of reflecting the most light, are the most brilliant of all the diamond styles. Fancy shaped diamonds—or any diamond shape that is not round—follow different proportion guidelines than the ones outlined above, and affect the ability of the gemstone to refract light in different ways. When light strikes a fancy shaped diamond, a small amount of light is lost through the bottom of the stone, creating an effect called a "bow tie."