Because diamonds cannot be scratched by anything but other diamonds, we often forget that diamond jewelry can rub against each other, if not kept separated in a jewelry box, and cause scratches or abrasions on the gemstones in touching rings. Toughness is another issue.
Diamonds grow in what are called cleavage planes.
These can be loosely compared to the way in which pyrite grows, layer upon layer. Because the area between these layers of carbon is not as strong as the solid carbons areas, a blow to a diamond at just the right angle can actually cause a chip in the stone. An existing inclusion or flaw can also spread if the diamond is hit in just the right way. Cutters of rough diamonds actually set up their stones for cutting, angling their tools to catch cleavage plains and cleanly break diamond rough. Always expect your jeweler to spend time with you before a diamond purchase to explain the necessities of a knowledgeable diamond choice.