Found this 9 months after it got posted, so this might be a 'captain hindsight'-level reply, but have you heard of Metalicoin?

Not just precious metals, but ALL metals that have industrial usage can be traded AND will be available for delivery (or at least, that is what they envision).

ICO should be sometime soon, but in the meantime you could check out their white paper to see if this is what you were looking for.