Cut is the most important of the four C’s because the brilliance of a diamond depends heavily on its cut.
Don’t confuse diamond “cut” with “shape.” Shape refers to the general outward appearance of the diamond, (such as round, princess, or oval).
Cut refers to the diamond’s reflective qualities. The quality of the cut does make a difference in how a diamond looks.
A good cut gives a diamond its brilliance, which is that brightness that seems to come from the very heart of a diamond. The angles and finish of any diamond are what determine its ability to handle light, which leads to brilliance.
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 reflecting back out of the diamond through the table and to the observer’s eye. This light is the brilliance we mentioned, and it’s this flashing, fiery effect that makes diamonds so mesmerizing.
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.
Less light reflected back to the eye means less brilliance.