Girdle Thickness
The girdle is the area between the crown and the pavilion of the diamond. It may seem like a trivial part of evaluating a diamond's proportions, but a very thin girdle can cause a diamond to chip. Although diamond is the hardest substance on earth, it will chip very easily if cut too thin. The girdle is located on the outer perimeter of the diamond and highly susceptible to damage if hit hard enough. The most desirable girdle thickness is "medium" or "slightly thick". Girdles that fall into the "thin" to "extremely thin" range do not significantly affect the value of a stone at the time of purchase, but the risk of damage is far greater.

Girdle Finishes
There are three common types of girdle finishes:

Rough Girdle
Has the appearance of frosted glass and is the most common. As long as the girdle thickness is correct this finish has no effect on a diamond's value.

Polished Girdle
Has a glass-like finish and improves the overall appearance of the stone. The rough girdle is noticeable when viewed from the side, the polished is not.

Faceted Girdle
This newest girdle finish is faceted like the rest of the diamond, adding as many as 50 facets to the girdle. Many experts feel the extra facets provide a little extra brilliance and a more finished look to the diamond.