IRON, COAL, COPPER, OIL and SALT lie in the land's special tiles.
IRON in the mountains. COAL or COPPER in the hills. OIL or SALT in the desert. OIL in the wetland.
You hold a deposit when one of your cities works its tile, or when you keep a camp on it.
WITHOUT THE MATERIAL, the work costs half again as much. It is a tax, not a wall.
SALT is a luxury too: holding it calms one citizen in every city.
Exactly what wants what:
COPPER: Phalanx, Chariot, Catapult, Trireme, Mech Inf, Cruise Missile, Reaper Drone, Nuclear, Hydro Plant, Observatory, SAM Battery, Neural Lab, Mission Control.
SALT: Granary.
IRON: Cannon, Artillery, Ironclad.
COAL: Factory, Power Plant.
OIL: Armor, Fighter, Bomber, Cruiser, Battleship, Submarine, Carrier, Transport.
Mine the tile you work, camp the tile you don't. Stand a unit on a rival's camp and it is yours.