Cut

The beauty of a diamond depends more on cut than any other factor. While all the other Cs are largely determined by Nature, cut is the one that can be determined by Man. For this reason, cut is perhaps the most important of the four Cs for us to understand.
Cut refers to the proportions, polish, and symmetry of a stone: these three factors are important in creating a diamond with the best possible light reflection. The angles and finish of any diamond are what determine its ability to handle light, and its 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, and it's this sparkling blaze that makes diamonds so captivating.
A poor cut, on the other hand, can cause light to seep out of the sides and bottom of the diamond, or it can limit the amount of light that enters a diamond. Poor cuts can therefore cause the diamond to appear dark, dull and lifeless, despite its colour and clarity grades. This is why cut is the most significant factor in the appearance of a diamond.

SGS Cut Grades Description
Ideal/ Excellent Excellent light performance. Reflects almost all of the light that enters. Rare and extremely beautiful cuts.
Very Good Very good light performance. Reflects almost all of the light that enters. Very Good diamond cuts are considered to be an outstanding value.
Good Good light performance. Reflects most of the light that enters. Good diamond cuts are far less pricey than Very Good cuts.
Fair Not as brilliant as a Good cuts or above, Fair diamond cuts are still considered good quality diamonds.
Poor Poor cut diamonds are typically cut too shallow or too deep causing much of the light to leak out of the diamond's sides and base.