The Ivory Chess Knight is the eleventh treasure that can be found in Uncharted 2: Among Thieves.
The Ivory Chess Knight is found in Chapter 2 - Breaking and Entering. Next is the room where Harry gave you the Tranquilizer Dart Gun. Before using the support beams to walk and climb to the staircase at the top end of the room, you'll first want to go to the right end of those beams, and climb up to what appear to be isolated and useless horizontal beams on that end of the room. However, when you climb up atop those beams and keep an eye on the windows, you'll no doubt spot the telltale glimmer of a treasure there. Simply walk over to it to grab it, and then drop back down to the previous beams, and work your way forward to the stairs.