Plastic pellets, these are sometimes used as a filler to cushion rock in ROTARY tumblers especially when doing fragile or harder to polish stones such as Apache Tears, glass, Rose Quartz and other things.  Use them in the pre-polish and polish to help prevent white edged poorly polished stones.  These pellets FLOAT, making it easier to separate from the rock when done.  They are reusable, although they will wear out.  Don’t mix them between the ones used to grind or those used to polish.

Not sure if these are your best option for filler media?