District 12's main export is coal. Here is a list of all of the Districts:
District 1: Luxury
District 2: Stonemasonry and Weapons
District 3: Technology
District 4: Fishing
District 5: Power (producing electricity and etc)
District 6: Transport
District 7: Lumber
District 8: Textiles
District 9: Grain
District 10: Livestock
District 11: Agriculture
District 12: Coal Mining
District 13: Graphite Mining and Nuclear Weapons