A United States Coin, prior to being minted, may go through several stages where a proof coin is minted. This process is in preparation for the minting of the final coin. While these rare US coins primary purpose is for checking dies and archiving, nowadays they are struck in greater quantities for coin collectors (numismatists) and can even be found as a Proof Coin Set.
Original text: Gold, Silver and Platinum Bullion Can Be a Great Investment