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.