Anna, my standard practice when sizing/soldering up or down with any
ring with stone[s] I have any doubts about as to whether they can
take any heat is -
I use a small baby food tin, packed to the top with fine white sand
and add enough cold water to wet the whole contents, drain off any
surplus water, then push the ring head/stone just under the surface,
then lay a thin piece of pumace onto the area above the stone- which
will soak up some of the water also.Then stand a pumace piece behind
the area being soldered to help reflect the heat back to the rear of
the joint to stop heat loss.
Just make sure the pumace and sand surface does not dry out too much
if you try this system.