What kind of rock is this?

[NathanD]

At its longest point, it's about the size of a baseball's diameter. Found it sitting on a railing while I was taking a walk on a river trail near me and thought it looked really interesting

 

[DDancer]

Welcome to the forum NathanD. What you have is slag, a byproduct manufacturing process's. If you like it keep it. A lot of have a bit or two because it looks neat.

 

[Chazhavok]

I keep them too because they are neat looking I see them in our area too!