Our 20mm Stainless Steel Magnetic Clasp is made of rust-resistant jeweler's grade stainless steel, then polished to a high shine. The 4mm Glue-in ends accept multiple strands of thread, cord and other stringing materials. Our 20mm Stainless Steel Magnetic Clasp is made of rust-resistant jeweler's grade stainless steel, then polished to a high shine. The 4mm Glue-in ends accept multiple strands of thread, cord and other stringing materials.