Open a large jump ring (approx. 8mm) and attach it to the crystal. Close it carefully to avoid chipping the crystal. For best way to open and close loops, see Jump Ring Hint in Jewelry Basics 101 (PDF).
Use a smaller jump ring (approx. 5mm) to attach the large jump ring to the ear wire. Two jump rings are needed to suspend the crystal from the ear wire in the correct direction, plus it gives extra movement to the earrings.