The Jade Pei Pendant is the eighteenth treasure that can be found in Uncharted 2: Among Thieves.
The Jade Pei Pendant is found in Chapter 3 - Borneo. After going over the ridge (which should be lined with dead bodies) and veering rightward, you can climb upward. To continue with the main quest at hand, you'll want to continue along the path on your left, but to find this treasure, veer off to your right during the climb. You may have seen the location of this treasure when you were down on the forest floor. It's located above the waterfall you past earlier. Unable to shoot it down from below, you can grab it from up here. After taking the right path, follow the shallow water to the apex of the waterfall. The telltale glow of the treasure will emanate from atop a severed tree trunk.