[runeks]

You could build an exchange into the game -- a simple bid/ask order book. And add the ability to use more than one currency in the game. Bitcoins could work alongside "Emeralds", and simply be traded voluntarily via an exchange mechanism. Although if people are able to mine Emeralds without some kind of limit, it wouldn't take that long for people with bitcoins to stop bidding on Emeralds, and bitcoins would taker over as the currency (I assume). Would be a very interesting experience regardless of what happens.

Of course, this concept of using real money in a game is fundamentally incompatible with the ability to create something out of nothing inside the game. If you can create something without effort, why would you ever pay for it with real money? If, however, items take time to create, people would be willing to trade the item for money. Presumably the item would become increasingly expensive the more labor (or, simply, "waiting") that is put into it -- just like the real world: an entire apartment complex is expensive because it requires a lot of work to build, and 30 year old Whiskey is more expensive than the same Whiskey at 5 years of age, simply because it takes longer to make.