Bell Cap gold-plated brass 4-prong 5mm, sold individually.
These are a great way to make the top of irregular beads look uniform. You may need to pry the prongs open a little to get your bead in, then secure with a dab of glue and let dry. Just make sure to glue the cap on in the direction you want the bead to hang. The last picture shows them in use and illustrates how they can be used on a variety of bead sizes.