Hiya CQ - long time no see. Hope you're well over there!
A tumbler (in this instance) is actually for metal barrelling - polishing metal with stainless steel shot and burnishing compound in water - the machine simply turns turns a 3lb rubber barrel slowly enough for it to 'tumble' the pieces together - the effect is like thousands of tiny little hammers bashing the metal gently to polish it.
If you've bought rings, they should already have been tumbled to deburr the cut edges and polish them, so provided you don't overwork them you can just buff the finished piece up with a polishing cloth (or a bit of very fine steel wool if you want a bit more shine).
For those of us who cut our own, a tumbler is pretty vital - and is handy for giving a final polish to anything that can go in it!
The other use is as a lapidary polisher - you put rough stones (gems or just pebbles) in with progressively finer grades of grit to bring them up to a smooth and shiny finish (hence tumblestones) - this can take weeks at a time, so a good quality machine is a must or the motor will burn out.
hth
george
