I have suggested holding copper like silver and gold, though I do feel that although their value will increase against the US dollar over time that these metals should be in products since that is where they have the most utility. I had imagined using their weight as a method of energy storage via gravitation. Though I would love to implement a more complex gravitationally augmented electrochemical energy storage, a generator, strings, and pulleys would add utility to your stockpile while you wait for their solid state value to rise.