Johnathan I am thinking worn rock. I am smiling cause I cleaned out my saddle bags on my 4 wheeler from a rock shelter dig. I had found some nut stone morters and had a pestle with them. The more I look at that pestle the more I am convinced they never used it though it is just like others I have but has no wear. Crazy.
I am thinking rock but maybe the pic is bad on my end so I will ask....is there a dent on it? And if there is a dent does it show any scratches or swirls in the dent?