I would like to see cuprite or porphyry be to copper what heavy earth is to iron. Copper is currently harder to come by for most people than steel, which makes little sense both in the context of the history of metallurgy and also from a gameplay perspective. Considering that men on the tail end of the Stone Age were able to scrape together enough copper from mining to make tools and jewelry, getting a little bit of copper should probably be easier than it currently is.
Like with iron, getting a lot of copper or high quality copper should be the hard part.