I'm curious about how hard a stone must be to survive a trip
through this tumbler.
Harder than steel (the pins). a warning though, if you put more than
one stone set piece in at the same time, then stones may be abrading
against other stones. In general, though, unlike traditional rotary
tumblers, and even some vibratory ones, the mag finishers are pretty
gentle, not delivering impacts to stones. So unless there’s
something in there that could scratch the stones, or if the settings
are so fragile that the pins could wedge in and somehow loosen stones
(I’ve not had this happen, but can imagine how it might…) you
should be fine. Try a test piece to be sure.