Toughness

Any stone, including a diamond, will fracture if it’s hit hard enough in the right place. Toughness is a measure of how well a gem can survive an impact and resist breaking, chipping, or cracking.

A gem’s toughness is a function of its structure. The way diamonds form affects their physical and optical qualities. When the atoms come together under extreme temperature and pressure conditions, they bond to each other more tightly in some directions than they do in others.

Diamonds are tougher in the directions where the atoms are bonded tightly together, and less tough where they’re not so tightly bonded. The weakest directions are the ones where the atoms are farthest apart. It’s easier to break a diamond in those directions, which are called cleavage directions. A cutter can cleave a diamond by hitting it sharply in the cleavage direction. But even after cutting, a hard blow can still cleave or fracture a diamond. This can happen during the setting process, or even when it’s being worn.

Sometimes the toughness of a diamond is related to its cut features and shape. For example, a diamond with an extremely thin girdle is especially vulnerable to damage. Also, some shapes are more prone to breakage than others. Pears and marquises, for example, have points that are vulnerable to damage if they’re not covered by prongs.