Easy to pick up and learn, tough to win.

I made the mistake of playing it like a deck-building card game, only picking up mines that had victory points on them. These were few and far between, though, so I ended up with a lot fewer gems to use to purchase the more lucrative mines that opened up later on.

Three things I learned about game design:

• Don't rely on just color to distinguish sides or types in the game. I'm color blind, and had a hard time telling two of the gems apart, because their colors were so similar.
• Even a rather simple mechanic -- gems buy mines, which give gems to buy more mines -- can yield an interesting game, once randomness and competition enter into it.
• Introducing an unbalancing element can be ok, if it pushes the game towards a conclusion