If it's a clean polish you want without the need for elbow grease, a metal vibratory bowl (or tumbler) should be on your radar. Not to be confused with a vibratory feeder, which Google will spout ...

[Fraens] decided to go with a similar electromagnet arrangement to what he used in the vibratory bowl feeder he built last year. Pulsing the electromagnet pulls down the large 3D printed spring ...