It is not recommended to fire your clay with the marcasite in place. Doing so can damage the stone. To glue it in place, you will want to make an indention in the clay piece for the stone. This indention should be slightly larger than the stone to allow for shrinkage during the drying and firing process (approximately 8% for designs made with Art Clay 650). Once you have fired your design and burnished the surface, you can glue the stone in place with jeweler's epoxy.