No one has mentioned the old-fashioned, low tech way to solder silver with a heat sensitive stone. So, I will:
I use a clean tuna can and fill it with clean, fine-grained sand. Got mine from a box of bird “grit.” Then add enough water to thoroughly wet the sand - not soupy though. After applying whatever flux you use, sink the heat sensitive bit until only the part to be soldered is exposed above the web sand. Actually, you could apply the flux at this point, if the joint is clean.
Now you apply the torch and solder. As the silver will transmit heat, you are likely to see steam rising - possibly some bubbling in the sand. This will take quite a bit of heat as much is lost to the wet sand, so expect to turn up the torch. Watch and quickly pull the torch off as soon as the solder flows.
I’ve sized many silver rings this way, not to mention soldering jump rings through sensitive stones. People who understand the heat on stone quandary always ask me how I did it.
I claim no originality on this method. An Orchidian, whose name I don’t recall described it in the dark past.
Judy in Kansas, who is wading into the dreaded paperwork associated with submitting income tax!!