Diamonds are considered to be the hardest of all gemstones, and in the Middle Ages they acquired the name of "the invincible" In a diamond crystal, however, hardness depends on direction. Some directions are harder and some are somewhat softer. In cutting only the softer directions can be worked on successfully. A diamond also possesses good cleavage parallel to the eight octahedron surfaces which limit the durability of the "king of gemstones".
A diamond reacts sensitively in these directions to pressure and blows. Small indentation marks (Figs. 333, 339, 341) or even indentation cracks running mainly along the directions of the cracks develop, in particular on facet edges and corners.
The
development of indentation marks and edge damage of this kind in set
stones can be caused by claws which are far apart, especially in the
area of the girdle, where nicks and fractures can form in varying sizes.
| Growth lines | Nicks and fractures |