Why create coins pegged to a particular commodities?  Instead, make a modified system of contract ownership that can be traded through a blockchain accounting system.  For instance, I issue a contract to ship 1 oz of silver to the holder of a contract I issue and then sell it for BitCoins through a block chain.  The owner of the contract can either trade it to someone else for a different type of contract or issue it back to me for redemption.  The block chain would use a similar accounting method to determine who the owner is along with added mechanisms to ensure trade takes place smoothly (e.g... one contract for another contract or BitCoins for the contract).