SPI Supplies Brand Agate Ball Mill Grinder Balls, 5 mm Diameter, Pack ...
There is a class of grinding apparatus called "ball mills", and are manufactured by firms such as Spex Certiprep, Glen Mills, Inc., Retsch and other fine ball mill manufacturers. The concept of operation is easy enough to understand: Balls of a particular hardness are tumbled in a closed container to erode the size of the material being ground.