The Tibetan Turquoise Ring is the seventy-eighth treasure that can be found in Uncharted 2: Among Thieves.
The Tibetan Turquoise Ring is found in Chapter 22 - The Monastery. This is a fairly well-hidden treasure, but one that's easy to obtain once you know precisely where to look. When you're climbing up to the area above the locked door, don't go rightward. Instead, use the bricks to swing around to the left side of the building. You'll see an overhang below, one that you can drop down on. From there, drop down on the balcony below that the overhang rests over, and look up. You can shoot the treasure down from its perch here and grab it once it falls. Then, use the debris and bricks on the left to lift yourself back up and over to where you need to be to continue.