Bell Cap silver-plated brass 4-prong 5mm, sold by the cap.
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.
Personally, I use these on the rougher teardrop beads especially when I want to make them more uniform looking. I also love these on beads where the top stringing hole has been broke off. A perfect solution and a Save! for the bead.