Grinding Balls are made from high carbon steel bars with the proper alloying elements. Bars are heated to high enough temperatures to make them sufficiently malleable to be cut and formed into nearly perfect spheres. Balls are then heat-treated to provide them the necessary wear and impact resistance properties.
After being used in mills, these balls are getting smaller and not heavy enough to grind the materials, they will then become scrap. But their elements stay the same and can totally be recycled.