The Yuan Dynasty Coin is the sixteenth treasure that can be found in Uncharted 2: Among Thieves.
The Yuan Dynasty Coin is found in Chapter 3 - Borneo. After activating the first of Chloe's explosive devices and gathering the detonation pad she left nearby, you'll be able to proceed forward through the dense, aqueous forest. Almost immediately on your left, you'll find a log leading upward to an isolated section of the path. As you walk up the log, you'll almost certainly see what you're looking for. The glint of the treasure you seek is on a tree branch. Use Nate's gun to shoot the treasure down, and then run up to it to claim it for yourself.