An interesting, mildly related tidbit is that certain types of seashells were used as currency in large parts of the world until about 150 years ago. [0] Prob

An interesting, mildly related tidbit is that certain types of seashells were used as currency in large parts of the world until about 150 years ago. [0] Probably occupying a similar sweet-spot: rare but not too rare and robust. An additional feature (or bug) would have been the inherent quantization. [0] http://en.wikipedia.org/wiki/Shell_money http://en.wikipedia.org/wiki/Shell_money