It is by nikit rock or something like that I cant remember the exact name look on your map to the north. You have to go up the mountain and there is a cave there with the treasure in it. If you have beat the game already I can give you another major hint but it has spoilers.
Weird, I found that treasure one of the easiest to find, and I'd only just entered West Elizabeth, and had never been to that cave before. It's just obvious by the trees that it's on a mountainside in Tall Trees, and you can look on the main map for the winding path leading up to it.