Dear Linda, genuine sapphires and synthetic sapphires will go
through the fire without any problems providing they’re not too
heavily included. Some inclusions have a different coefficient of
expansion to the surrounding material and this could cause
fracturing. If the sapphire is free from inclusions (a little
"silk" in the stone won’t matter) just make sure that the stone
is clean, no fingerprints or dirt left on the stone, and be
careful not to let any flux burn onto the stone while you are
soldering. Some fluxes will etch into the surface when heated.
Your mention of a yellow sapphire sounded an alarm bell which
began with my own experience some twenty years ago. I had a
parcel of what I thought were beautiful golden-orange sapphires.
We used some in a ring which had to be resized and blithely went
ahead just as we always had done, doing the resize with the
stones in place.
As the ring was heated, all but one of the sapphires went
transparent white. So did our faces! We immediately took action
against the Australian dealer who was as horrified as we were.
The matter was eventually traced back to a Sri Lankan dealer
whose brother was a radiology technician. They had been dosing
white sapphire with heavy X-ray bombardment to turn them a
beautiful deep (and highly expensive) gold colour. Trace
elements in the white sapphire respond to X-ray irradiation. It
only took a little heat to dispel the colour - about boiling
I’m very careful with golden sapphire if the provenance is
uncertain, and insist on warm-testing such material before
buying. I don’t want to frighten you, but it happened to me,
so… Hope this helps, Rex from Oz