Girdle is the thin perimeter of a diamond, dividing the crown above from the pavilion below. When viewing a diamond in its setting or from a profile view, the girdle is the widest part (or the circumference) of the polished diamond - the portion of the stone that makes contact with the setting itself. When loose diamonds are measured, they are measured by the girdle to obtain length and width in millimeters.
A diamond girdle can be rough, polished or faceted. Brilliant cut diamonds (with triangular-shaped facets) usually have a faceted girdle rather than a perfectly round girdle; this gives the gem a higher measure of transparency. Step cut diamonds (with rows of elongated facets) usually have polished girdles that are not faceted.