Sorry, I don’t believe there is any substance that will protect the
stone from the heat required to solder silver right next to the
stone. In the days before lazer welding and pulse-arc welding, I
would quote to cut off the bezel to remove the stone, solder the
shank, then restore the bezel and set the stone as it was. A huge
Pulse-arc welders are a godsend for situations like this. Pulse-arc
has more power and works better on silver. Welding is not always as
good as solder, but in this case, welding is the best way.