I've read that people who make jewelery have small tumblers that they fill with rocks and abrasives. They leave these small machines turning for hours and the rocks get polished very smooth.
I've seen a cement mixer that got very clean after it was run for a few hours with some pieces of 2x4 and pecan shells in it. (The sound of the machine running was rather loud.)
Are there any other interesting examples of tumbling being used to polish or grind up material? I'd like have some sort of small machine that would pulverize old pieces of concrete and concrete block into particles that could be re-mixed into crude concrete construction like concrete poured around fence posts. (However, I don't know if I'd want to listen to such a machine for hours and hours.)