What tumbling media and filler actually do, and when to use them

Plastic or ceramic tumbling media (small pellets or pieces added alongside your stones) serve a few genuine, practical purposes. They help cushion small or fragile stones from directly banging into larger, harder ones, they help fill empty space in a barrel that is not quite full enough for good tumbling action on its own, and specialized plastic pellets are commonly used in the final burnishing stage specifically to add shine without any abrasive grit at all. Filler is particularly useful when tumbling a small or oddly sized batch that would otherwise not have enough material for the stones to tumble against each other properly.