Hello. I am new to this forum. I found something that might be a cannonball buried in backyard. It is heavy weighing about 6 pounds and it is 12 inches around. A magnet does not stick to it and it shows signs of corrosion. In my research I have also learned it could possibly be a mill ball. I live in Pittsburgh, PA. Is there a way to determine if this is a cannonball? Thank you.