The Spanish Silver Coin is the twenty-fourth treasure that can be found in Uncharted: Drake's Fortune.
The Spanish Silver Coin is found in Chapter 6 - Unlocking the Past. You can grab this treasure after unlocking the door with the keys and fighting the swarms of pirates. Head up to the large gate at the far end of where the gunfight took place. Face the left side (the side with a giant white tree). There will be a small pile of rocks there. Climb up these rocks and hop over the ledge for the coin.