Kevin Whitmore and Judy Anderson at Rio Grande have done extensive
experiments on what gemstones can withstand the heat of being
fired in PMC3 at 1110 degrees.
You can find the list of stones on line at:
"Stone fired in place into PMC"
Go to Tech Resources for 2002 Conference, and it's the 2nd one on
He lists which stones survived, and which ones survived with some
Seems like a good place to start.
Metalsmith, Certified PMC Instructor
Hard to Find Tools for Metal Clay