pixel_player35957100 look for unopened treasure chests and tiles that blend in well with their surroundings.
Another trick I find useful is to get Diggy to walk around next to everything, as that will light up a tile as you get in its vicinity. Simply mousing over you might have missed something and your mouse won't have the effect that Diggy's presence would have.
But there is probably some small room you have forgotten to check that has all 12. Go back to the very beginning, to where you tap on a point of entry that asks Do you want to leave this location? Start here:
Here is an example of what I mean about moving Diggy around. He is several steps away from the sapphire tile
Now he is two steps away but you are alerted that there is a tile of 33,000 to be clicked and it's in red because I don't yet have enough energy lol: