Oh it has a name....
I know what its called but its really racist... no seriously. They are called (racial term for African folks)heads. There is a quarry near me that pulls them up all the time. They really need a better name, but it comes from a time when that was a common term. The guy that told me about them is dark skinned himself, which makes it seem worse. They are basically just Iron balls, most often formed by soil movements and, in your case, discarded Smelting material.
You have the left overs of smelting metal. There are a few veins that drain the top most layer of material off a smelting pot. Mostly zinc, tin, and other additives to the real prize, Copper. When the "scrap can" is full, it is hit with a hammer and out comes a cylinder of mixed metal.