I've found every stones but the L. I may have missed a room, but I can't find which one... Is it in the spiky room (castle)? I think it's the only one I didn't entirely get through for now.
From the candy box 2 wiki
The L stone is hidden in the chest inside the upper room in the castle area. Inside this room, you can see the chest at the end with a lot of eggs in between and a monster floating near the ceiling. Walking up to the chest and breaking any of the eggs will cause the monster to instantly rush and kill you. The solution to avoiding the eggs should be pretty obvious.