I believe I see a definite patina difference between the "ring" between the hump and the outside edges. The hump and outside edge have a green tint to them where as the inner ring is different.
The last set of pictures of the edge view seem to show a dished out middle in the hump.
The outside shape of the stone resembles some manos I have
Being found next to a stone wall, it could have been a mano that was then altered by someone during the building of the wall. 18 and 19 century settlers used stones also for different things.
I'm not saying a native American made it in the first place but it does, in my opinion seem to have been altered by man but for what purpose I have no idea.