The Silver Dress Clasp is the twenty-fourth treasure that can be found in Uncharted 2: Among Thieves.
The Silver Dress Clasp is found in Chapter 6 - Desperate Times . This is the first of six treasures found in this chapter. Once you've finished all waves of enemies, search the facade of the building in front of you, the temple-like structure where enemies were pouring out of during the previous firefight. Above the main door, you'll see some sort of emblem (seemingly like two fish) with a gem in the middle of them. That gem is actually the treasure you seek. Use a gun of your choosing to knock the treasure down, and then run up to it and grab it. Just like with the last treasure, though, you'll want to wait until the firefight ends before grabbing it.