An individual stone, either natural corundum, lab grown corundum, or cubic zirconia. Push them into the wet clay, and fire with the stone in place. Beautiful stones - they are top quality cut.
All of our kiln safe gemstones are test fired up to 800°C
I have bought this fireable gem, lab. created (Peridot), in various sizes. It maintains it's colour very well when fired even @ 880C.