The Yak Horn Carving is the seventieth treasure that can be found in Uncharted 2: Among Thieves.
The Yak Horn Carving is found in Chapter 20 - Cat and Mouse. After your first encounter with the tank, Tenzin will be fumbling like a fool to open the door. After entering the house you will kill a few more guys and round a left corner where there will be more enemies on the roof. BEFORE you need to jump over the wall the treasure is just to the right near the AK ammo.