Iron

Iron (also known as steel) is a very valuable resource on Earth-like planets, even in the space age and can build many strong things. Your game will prosper once you develop the needs to aquire iron, as it can build very strong structures and things, so they are great warfare materials. Iron is much more rare then Stone, but can be found in Stone rocks, mines, and even more uncommonly, by themselves. Iron is best stored in warehouses. Iron can be a game's 'engine' or it's end, can you decide?