Consider applying a small amount of Poly Bonder® to the clay, then apply the crystals to the adhesive. Poly Bonder is intended for use with polymer clay and high-temperature applications.
Depending on how you "set" the crystal into the clay, you may find they do not stay seated once the polymer clay has cured. If the crystals do come loose, apply a small dab of two-part epoxy to the area where you want the crystal fixed in place, then set the crystal on top of the epoxy. Clean up any excess glue right away, as it will leave a shine if cured in place.
- Tammy Honaman, Author, Jewelry-Making Expert and Educator