In the recent thread on crack in 4.7k diamond, Peter Rowe made the
comment that diamonds are "quite tough."
Toughness in stones is an interesting topic, and the archives have a
nice thread on the subject that took place some time ago. Perhaps it
is time to revisit this subject.
Diamonds are unquestionably the hardest substance on the planet but
as for toughness, there is some difference of opinion.
In addition to being hard (or perhaps because of it) diamonds are
notoriously brittle--the crack in the 4.7 carat stone is a case in
point. On that dimension at least, diamonds are not particularly
tough; especially vulnerable are the girdles.
Jade is a stone that is unquestionably tough, due to its particular
structure, but at the same time one of the softest materials.
So hard doesn't mean tough, and soft doesn't mean fragile.
Some of the gemologists may want to contribute more