There are a limited number of mining locations in the world.
Mine - Upfront: 20,000 gp - Labor: 50 miners (182,500 gp/year, 18,250 rations/year) - Annual yield: 550,000 gp of ore x 0.8 each year (yield decreases by 20%) - 10% chance of finding valuable gemstones, including diamonds
Ore Refinery - Upfront: 50,000 gp - Labor: 30 workers (109,500 gp/year, 10,950 rations/year) - Converts all ore into iron, doubling its value - requires 1000 acres of forest for lumber or equivalent power source
Steel Plant - Upfront: 100,000 gp - Labor: 20 workers (73,000 gp/year, 7,300 rations/year) - Converts all iron into steel, doubling its value - requires 500 acres of forest for lumber or equivalent power source
Workshop - Upfront: 5,000 gp - Each craftsman in the workshop converts 50 gp of steel into 100 gp of valuable goods (e.g. weapons, armor, etc.)