If copper tools were to me implemented into the game, and seeing how it's as abundant as it is, only being less abundant as stone. Copper tools should have the same, or similar, effectiveness as stone tools, but with a higher durability. The jump from stone to iron tools is extremely odd when you add a metal such as copper that has been used in a historical sense. Maybe they're holding back on the idea of copper tools because then they would have to add tin to make bronze. Considering the oxidation of copper tools, I think it would make sense that they wouldn't oxidize with continued use, or while being stored, and the fact that blocks of copper only oxidize if they are placed in the world only adds to that.