This is very technical! A tuna can filled with fine sand that is
saturated with water does the trick. Sink the gemstone and about
half the shank in the wet sand before soldering. Be aware that you
will need to turn the torch up a few notches to overcome the loss of
heat - it dissipates quickly in the wet sand.
You might experiment with the technique by simply soldering a band
closed when half of it is submerged in the wet sand.
Good luck, Judy in Kansas, where we've had three, count 'em - THREE
days of gentle rain. So nice, and everything is blooming and