The Tibetan Knife is the fifty-first treasure that can be found in Uncharted 2: Among Thieves.
The Tibetan Knife is found in Chapter 15 - Train Wrecked. The last treasure can be found on the right hand side from where you started.Once more, you'll want to wait until all enemies are killed before you go searching. Head along with the the cliff side on your right and you should see this treasure flashing in the rocks directly in front of you as you approach. (another point of reference is to the right of the train cart that exploded after the last enemy wave).