Round Black Elastic Cord Stretch Elastic Bead Cord Crafts 1mm/1.5mm/2mm/2.5mm. Black/White Round Elastic Cord for Face Masks Beading Crafts 1mm/1.5mm/2mm/2.5mm. The extendable cord is made of elastic which makes it stretchable, durable and sturdy enough for long-run.
eBay