I think coal is more likely, at least from the start. Not sure about the purpose of copper and tin, since bronze is worse than iron, unless ME adds another tier of tools right after stone. Coal would enable you to make steel weapons and tools and be used as a fuel. If food is going to spoil with time, salt could be added as a preservative. Salt mines were incredibly important in the middle ages. What else... Gold and silver? Maybe to pay taxes with them. I don't imagine the engineers making jewelry.

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