Finite deposits so that your mines dry up after awhile and require perpetual exploration and expansion. Also, limitless sinks for your metals and materials so that you should never have massive excesses to trade away. I imagine both of these factors play heavily into the scarcity and distribution for trade.
Reaper, Reaper, that's what people call me! Why?
Cause they all die! When I sing, I end their lives
You act as though payback makes you a noble man
Is that a fact? Well, you're a goddamn Philistine!