It's true: Diamonds are the hardest of all gems, ranking 10 out of 10 on the Mohs Scale of Hardness. They're more resistant to scratches than any other gem, but that doesn't mean they're invincible.
These photos are perfect examples of why a diamond's proportions can be so important. This old European cut diamond has a shallow crown and very thin, knife-edge girdle. Besides not having much brilliance, it also has a durability issue. The thinness of the girdle makes it more susceptible to chips and/or breaks.
As you can see in the photos, there is a conchoidal fracture that runs from the girdle, down into the pavilion. The break was most likely caused by the jeweler pressing down too hard on a prong while setting the diamond.
So what now?
Well...as is, the diamond weighs 0.69 carat. In order to repair the break, it will have to be recut.
The good news is it will be recut to modern round brilliant cut standards, which will make it much more attractive. The bad news is the diamond is going to lose a percentage of its size during the recutting process. After it's recut, it will finish as a modern round brilliant cut somewhere around 0.55 carat.
So what's the moral of the story?
When shopping for jewelry, take the time to examine the proportions of the diamond or gem, and how it's set. Does it have a knife-edge girdle? Does it have exposed corners, leaving it vulnerable to chips or breaks? It's always better to notice these issues before spending your hard-earned money on a potential problem.