Stoppers, or block beads are lined with a tiny rubber like "o" ring. When added to a bracelet, they will "stop" the beads from moving along the chain.
The Uptown Shop highly recommends adding a stopper as the last bead on your bracelet chain before attach the clasp or using a safety chain with your bracelet as a way to insure your beads stay on your bracelet in the event that your clasp is opened on the wrong end.
Many of our customers have come in to thank us for this recommendation.