I don't know much about artifacts yet. It kind of looks like what we use to use to dress grinding wheels! I'm sure I'm way off, but that is what it reminds me of!
Welcome to T-net, looks like you may have found a "bar weight", they're usually made from slate or some kind of hard stone like granite even jade. In your pics. it looks like some kind of hard stone to me. If that's what you have there, it's a nice find. Some more pics. would be helpful, views of the edges and straight on end views. Hope that's what you've got there, thanks for pointing it, good luck.
Bar weights are thought by many to be a tie on type of atlatl weight.
Looks less convincing now with more pictures, square rather than rectangular through the length of the piece. Bar weights are usually more rectangular through the length and draw down thinner at the ends although there are lots of variations. Could just be a work of nature but still looks like it has artifact possibilities to me, like stone chisel with broken tip or something but hard to tell with pictures.