OK guys, i think i found an answer (kind of) to this problem - there is no answer.
Following this guide:
my friend managed to get here:
(excuse the crappy quality, he took the pictures with his phone) which, on the map, is here
Notice that this is directly behind the rock wherein the 12th mosaic piece supposedly lies. Through mounting and dismounting, he actually managed to get INSIDE this rock, and we expected to easily pick up the last piece, as shown here:
http://i.imgur.com/KsxVjCW.png
Here's 3 pictures of him inside the rock
Now, upon entering the inside of the rock, his character got either stuck, or glitched out and fell down, flew into the air, and other such things which characters ought not to be doing. However, if none of this happened (he had to reload a few times) he would be able to stand still and click the search key, yet nothing would pop up - that's right, there was neither a ping nor a highlight of the mosaic piece. When going with the cursor over its supposed location, we couldnt see it, click on it, or make out anything whatsoever to stand out from the dark background.
Our guess is that even if the moasic piece is there, it exists as a static environmental prop, not something you can interact with. So if it is any consolation, no matter what you do, even if you enter the very bugged rock itself, you CANNOT pick up the mosaic piece.
