The Three-Stone Coral Ring is the forty-third treasure that can be found in Uncharted 2: Among Thieves.
The Three-Stone Coral Ring is found in Chapter 12 - A Train to Catch. The fifth and final treasure of this chapter is no doubt the most obscurely-hidden. It's located in the trainyard's workshop, and you absolutely, positively don't want to hit the lever on the train car before you obtain it. The idea is to get to the second of the three train cars in the warehouse, the one in the middle that you have to jump and shimmy over to. Once you're on top of this train car and begin walking back towards the entrance to the warehouse, keep an eye on the wall above and to the left. There's a small obtrusion there with the treasure on it; you should be able to see the glimmer from afar. However, to get it down, you need to use the red pipes leading towards it, shimmying over to it and claiming it in the flesh.