Hey United
You post a lot of stuff. While most of it is natural, you need to look at context. Was there a site near by? Was there anything else found along with the "artifact"
Remember that not everything can be identified via a picture. There are some fine details that can only be observed in person.
Get yourself some artifact identification books. Compare and see what you can find.
Stones span all sizes and shapes. Just because it look like a tool, doesn't mean that it is.
Do some research, it will help you.