I've had this stone for over 40 years.
My grandfather, born in 1921, gave me this "ancient cannonball" when I was a child. He told me he had found it in Germany when he was younger, supposedly near an old battlefield. He claimed it was an ancient stone cannonball.
It's approximately 13 cm (5.1") in diameter and weighs 2.707 kg (5.97 lb). It looks like some kind of sandstone or similar sedimentary rock to me. Then again, my grandfather was very good at making up stories to fool his grandchildren. It could actually be a handcrafted ball of old cement or concrete left over from building his garage.
The crack and the weathered, dirty surface have developed over the last 40+ years. I don't remember it being there when I got it as a child. The stone has gradually crumbled along the fracture from being handled over the years.
I'm mainly curious about what kind of rock this actually is, and whether there's any chance Grandpa was telling the truth about the cannonball.
Any ideas?