I don't know ANYTHING about Native American artifacts but I have been to places where rocks like that can be found naturally, so I don't know, but it could be natural.
the flat sides are interesting to be sure but it doesn't show any wear on the ends...no pounding, no missing material etc. I too have found similar stones (no flat sides however) along the PNW coast that are NOT pestles...same rock type too. All the same, it would have come home with me also.
mojjax......I've been looking at that avatar for a good while!! Still makes me laugh every time I look at it,reminds me of my brothers 50+ yrs. ago!! Anyway,I'm going to have to agree with ToddsPoint I like it.