You take the piece of thirteen to a rare coin dealer in Seaside Town (he's got a shop set up right next to that library across the street from the Sleazy Back Alley) to see what you can get for it. Turns out you can get X Meat for it.
When using with 0 adventures, you receive the message "You consider taking the piece of thirteen to a rare coin dealer to see if it's worth anything, but you don't really have time."
When using with no HP, you receive the message "You consider taking the piece of thirteen to a rare coin dealer to see if it's worth anything, but you're feeling pretty crappy right now."
The description matches the cursed Aztec gold coins in the first Pirates of the Caribbean movie.
The name of the item is a reference to the Spanish Dollar, also known as a "piece of eight" because its worth was eight reales. Presumably, the thirteen is a result of the cursed nature of the coin.
