Marcy, while the sapphire will withstand the temperature reached in
a gold casting, there are opportunities for problems to arise.
Please note that your instructor said "discolor", not change the
color. What he or she is most likely referring to is for a borax
coating to form on the surface of the sapphire and case a milky
swirl look. I have had this happen while re-tipping a sapphire in a
ring. If this happens, the only recourse is to have the stone
Other issues with cast in place are the speed which the flask is
heated and cooled. A rule of thumb is about 50 degrees per hour for
heating, and then after the pour, place the flask back into the kiln
and let it cool back to room temperature with the kiln, over a
period of a few hours. If you quench the flask as with an all gold
or silver casting, you are sure to crack the stone (sapphire, ruby,
or diamond). One other think you might encounter is that inclusions
in the stone may "POP" the stone during the heating stage.
The cast in place process does have it's rewards though and does
offer come creative stone setting ideas to come into life.
Perhaps others on the list provide more and tips.