This is one of my favorite little things of the game, and both the bane of my existence. The one in the Hinterlands can't be completed, the one in the Western Approach can be impossible to complete if you leave the area after fighting the Dragon WITHOUT picking one of the pieces up, which that fight unlocks.
And also for some reason they didn't bother to put something in the map like 'here was a mosaic piece' so if your memory fails you like me you have to go to back to every, single, one, and since there's all sorts of bugs with them at the moment its possible you didn't find it, but its not there anyway.
210 hours over 1.5 playthroughs and I have never seen one completed and I doubt I will.
[Edited by OnlyLagKillsMe, 1/2/2015 6:18:53 AM]
You can actually get the Western Approach one if you leave the area where you fought the dragon.
Little secret. If you go with a warrior and grappling chain the rock, it will break it open and you can go get the piece.