Cut is not the shape of a diamond - it refers to the

arrangement of a dimaond's facets.

The cut of a diamond--its roundness, its depth and width, the uniformity of its facets--all contribute to a diamond's beauty, brilliance, value and durability.

The width and depth have the greatest effect on how light travels within the diamond, and how light exits in the form of brilliance. The stone on the left is too shallow, and the one on the right is too deep. The center stone is a good cut.