A good cut gives a diamond its brilliance. The angles and finish of any diamond are what determine its ability to refract and reflect light. The following diagram illustrates the percentages and angles excepted as "Ideal" diamond proportion.

As you can see in the images below, when a diamond is well-cut, light enters through the table and travels to the pavilion where it reflects from one side to the other before returning back through the table of the stone. The more efficiently a diamond is cut, the more light will return back to the eye. The diamonds that have the most brilliance and dispersion and scintillation are the ones that are cut the best. Quite simply, the diamond that is cut the best will look the best.

In a poorly cut diamond, the light that enters through the table reaches the facets and then 'leaks' out from the sides or bottom of the diamond rather than reflecting back to the eye. Light that leaks out of the pavillion facets causes the diamond to appear dark and lifeless in that area..

Don't confuse diamond cut with shape! Below are various SHAPES of diamonds.