There seems to be some discrepancy between the XML game files and what happens in game with the Coal and Aluminum resources. In the game files, the numbers for the production yield for the resources and improvements show Coal and Aluminum behaving exactly the same as Iron. However, in game, a mined hill with Iron yields 5 hammers and a mined hill with Coal or Aluminum yields 6. Why is this? Does it have something to do with the fact that the ResourceClassType for Iron is RESOURCECLASS_RUSH and the ResourceClassType for Coal and Aluminum is RESOURCECLASS_MODERN? There is no mention of this in the Civilopedia either. I have looked for an answer to this one in the forums, but I figured if anyone would be able to answer this it would be you.