These are hollow metal beads that have been sliced open down one side. This allows you to slip the bead around/over your crimp and give it a little squeeze close (I use the outside part of my crimping pliers - it works a treat! But you can use regular pliers, but be gentle!). Hey Presto - it looks like a regular 3mm bead suspended on your stringing material! you simply slip them over your finished crimp and squeeze them shut.
Size: 3mm (will fit over a 2mm crimp comfortably)