John Holbo talks about a movie experience with his daughter, and he mentions that his daughter proposed a geode-based economy for her fantasy kingdom. I'm sufficiently ignorant of geological terminology to wonder what a geode is, so I looked it up on Google and eventually found, well, a geode-based economy. Or at least someone selling break-at-home geodes. Yes, people actually pay for rocks to be delivered to their home so that they can break them. Sounds fun.
(NOTE: I am IN NO WAY associated with the sellers of this product. Really. No, I mean really, I'm not.)