Hello, First post here, hoping it's in the correct location. I've had this object for years, and have always wondered what it might be. I've searched extensively online the last few days with no luck... So, about 15 years ago as a kid, I was on a sand/rock bar in northern Wisconsin...
www.treasurenet.com
A person in the Reddit post suggests it could have been a property line corner marker. Maybe your rock could be one, too.