pure speculation...Maybe it was used to mash up like indigo or some kind of dye then they could tilt it and the liquid would pour out the little spout there. Thats my best guess haha. Awesome find!
Need more pics like the bottom and sides. Could be natural or could of been used more pics will tell.
Natural? Hah
I'll try to get moe pics on here soon.
I would think it's NA, just because of the bottom, it's not flat and would've sat nicely on an earthen floor, whereas I would assume colonials would've likely given it a flatter bottom to sit upon a desk, etc.
As to it being natural formation ( minus tool marks), I wouldn't think so, but certainly far stranger geofacts are out there.
Thanks for sharing. I personally feel long past the "is it native" question. It's the "what is it" that is bugging us. Could bug us forever I guess..