Your piece has an edge on it like our Stone Hoes from my area. The Hoes we find though are made from a very dense and hard stone we call Waverly Sandstone.
Here's a few pictures of the Hoes we find here. You can see the similar flaking on the edges, but these all show lots of polish from extensive use. I have a feeling your piece may actually be part of a Slate Roof Tile and not and artifact at all.