It's inside silicone grommet has a 4mm opening that will securely hold the cording you thread through it. The opening will best accommodate our 1.5mm leather, our 2mm Waxed Cotton, and any cording from 1.5mm to 2mm thick. The outside rondelle is made of naturally hypoallergenic brass that is electroplated and nickel free. Basically, a string-on clasp is a stopper bead and the soft silicone grommet won't damage any of your jewelry making materials. They are most popular for replacing the macrame knot and perfect for adjustable necklace and bracelet designs.