Diamond is the hardest substance known to man. They are as good as new, if treated with care, even after years of wear in a piece of jewelry.
But be careful! Diamonds are quite vulnerable at the widest, face up round part of the diamond, known as the girdle. At times all it needs is a little blow on a hard surface like a granite kitchen top and your diamond can be chipped. Hardly ever a diamond breaks in two. Usually chips are rounded and cone shaped and part from the girdle or from the point on the bottom part (pavilion) of the diamond.
Usually it is difficult to repair a diamond. Most of the times a diamond has to be recut completely resulting in significant weight loss and high cost.