I think your idea sounds really interesting, but definitely some balance issues. In addition to what rdonj said, I also wonder if it wouldn't give an extra advantage to blood. Because your gem-gen summons would only produce 1/gem per turn, but many magic sites produce 2 or more, players in this game would have overall lower gem income than normal, even though they have higher than normal income in certain paths. Blood income wouldn't be affected, though, so it would be proportionately higher than gem income in your game.
Or at least that's what I think
. Does that make sense?