Renewable materials without using Mines?

My dilemma is this: I am attempting to create an objective / faction server, and I want to stay away from using resetting mines for all the different ranks, but I have no idea how else to get my players a renewable source of iron, gold, diamonds, etc.
How would you feel about trading emeralds to custom villagers for all your resources? You could mine them from the world, the 1 global mine, and get them from protected villagers across the world.
Can anybody suggest some good ideas, or give feedback on the villager idea?