You do not say what state you found this or in what context.
Was this a field find? Was it found along a waterway? If so have other relics been found there?
These specifics makes a difference.
From the image with the lighter the relic is relatively small. For me, in the mid-west, this is pretty small for a mano, pestle or food prep item. Could be, but I have my reservations.
Do you see any peck marks on this item or curves which terminate where the grinding plane would be?
If you do not find any peck marks or use wear, I would theorize its likely a natural weathering of the stone.
Some smaller relics about that size often will have a pecked and somewhat polished dimple in the center. I think those were used with bow fire starters but there may have been another use. (I have posted some of these in the past)
Here are two pestles I found on sites where I have found many other relics have been found over the used as a grinding manos at some point. I tried to take photos which shows the peck marks on the relics both on the ends and along the sides. Note that the curvature of the relic terminates in an angle where the curve meets the flat grinding plane.
Second relic, you can see the flatter side on the right.
Some peck marks, some large plow strikes on it too.
It was found in Massachusetts at the shore of a Tidal River, north of Boston. A river that emptys into the ocean but is affected by the tides.
Not a field Find.
The peck marks are visible but really ground down. I agree it does not look like a normal pestle or Mano, however the discoloration and Patina is really visible in the pictures. And it is worn and smooth all over. In the photo at top left, you can see where the curve terminates on the flat plane. I would love to find a matching metate and will try to find that.
It was found in an area that has a long history of native american occupation all over the town, near Salem MA.