I just found something today made out of the same stone I think, but I can see where mine was altered by man, check out my post from tonight. What part of the country are you in.
We've had it for years. We thought it looked like a duck (we live on Long Island and found it on our property we think) and have been using it as a paperweight.
We also found a projectile point on our property in the spring after pulling out what remained of a holly bush after Hurricane Sandy. That's what made us wonder about the rock.