The Silver Belt Buckle is the tenth treasure that can be found in Uncharted 2: Among Thieves.
The Silver Belt Buckle is found in Chapter 2 - Breaking and Entering. This one can be found in the fountain in the next vast, guard-filled room. Getting the treasure is rather easy, since all you have to do is jump over the fountain's outer wall, and run through the extremely shallow water until the on-screen prompt shows up, indicating that there's a treasure nearby. Specifically, the 2nd guard you have to stealth take out is leaning on the fountain, and the treasure is right behind him. Of course, don't try to get the treasure as you sneak up behind him, because he will catch you. Take him out, grab it, move on.