Coal pulverizing balls.
Fullsize image: https://diqn32j8nouaz.cloudfront.net...s_fullsize.jpg
https://diqn32j8nouaz.cloudfront.net...izer_balls.jpg
Previously:
https://www.homemadetools.net/forum/...737#post124083
https://www.homemadetools.net/forum/...854#post102669
https://www.homemadetools.net/forum/...875#post102727
