gir·dle | ˈgər-dᵊl
The outer edge or periphery of a fashioned stone, the portion that is usually grasped by the setting or mounting, the dividing line between the crown and pavilion, or the rim or edge of the diamond
The girdle is the outer edge or band of a faceted stone. It is the largest diameter of the stone, and is typically the portion of the stone that is held by the setting or mounting of a piece of jewelry. The girdle serves as the dividing line between the pavilion and crown. While the girdle is most commonly used for the setting, it varies depending on the cut of the diamond and the setting of the piece. The girdle is crucial to the care of the diamond. If it is too thin there is a chance of chipping in the stone, and if it is too thick, the stone may look smaller from the top than it should based on its size and weight. An overly thick girdle can also impact the brilliance of a diamond.