Did you know that the term diamond cut does not refer to the gemstone's shape? Sure, you have to cut a diamond to make it a shape, but when gemologists say "cut," they are talking about a diamond's proportions, such as its depth and width and the uniformity of its facets -- all characteristics that control brilliance, durability and other features we look for in a diamond.
A good cut is essential to a diamond's beauty, because even a diamond with outstanding color and clarity will not display the sparkle that diamonds are famous for if its components don't interact with light as they should.
A good diamond cut has many characteristics:
Diamond Width and Depth
The proportions of width and depth have a large impact on diamond brilliance, the reflection of white light that we see when we look at a diamond. Refer to the graphic on this page as you read the following descriptions.
Light traveling through a shallow cut diamond is lost out of the bottom of the stone and does not bounce back into sight. The lack of light play makes shallow cut diamonds appear lifeless.
Light traveling through a diamond that's cut too deep escapes out the sides of the diamond, darkening all or portions of the stone.
Light traveling through an ideal cut diamond bounces back out the top of the stone, bringing its brilliance into view.
The graphic illustrates extremes. As with other diamond characteristics, there are are in-betweens for cut quality.
Symmetry is a term that refers to the alignment of a diamond's facets, its flat and polished surfaces. The facets should be cut to achieve the best play of light.
You've seen diamonds flash when you move them in the light -- that effect is called scintillation, and it occurs when light bounces among the facets. Light doesn't reflect as it should if facets are misaligned, diminishing the fireworks display.
Diamonds graded Fair and Poor for symmetry usually have facets that are misaligned enough to affect the play of light.
The surface of the facets should be smooth and polished so that light can pass through them.
Common Problems with Diamond Cuts
A broken or chipped culet.
A culet that's missing or one that's off-center.
Misaligned or extra facets.
A girdle that is too thick, creating poor proportions, or too thin, making it easier to damage.
A fringed girdle, with tiny cracks going into the diamond.
A table that slopes to one side.
Always buy diamonds from a reputable jeweler and ask questions about diamond characteristics. Inspect as many diamonds as possible so that you can make informed comparisons.