It sounds to me like a rolling mill ball used crush the ore as has already been stated. We used to have a cement plant not too far away from where I live in MI. They used iron balls in their rolling mills.
As far as lead and a coal mine - I'm not a coal miner but i would suspect that if they used led balls in a rolling mill it would be to prevent sparks which could easily ignite coal dust - lead balls don't spark.
While lead cannon balls might have possibly been used at the onset of cannons, I highly doubt that what you have is a cannon ball. As mentioned, transporting lead cannon balls would certain de-form them to where they would not go down the bore. I have shot a number of Civil war cannon (live fire) - and we had a 10 pound smoothbore Parrott (South Bend Replica) - we cast and sued aluminum round balls in it. Any slight variation in "roundness" could cause it to hang up in the bore when introducing the projectile in to the bore.
And, I agree - if it was used for ballast or weight, cast in square or rectangular shape would be more workable and decrease the space needed for the placement of it. One might think of a "dual purpose" - i.e. cannon ball that could be melted down and cast in to balls, but again, that doesn't seem feasible. It would make more sense that they would use lead for a dual purpose such as was used on the Lewis & Clarke Expedition where sheet lead was used to construct waterproof gunpowder containers and once the powder emptied out of them, they could be cut up and cast in to balls.
Interesting though and I hope you get it figured out.
