Approximately 4x4.5x1.2mm copper plated Size 1 Wire Guardian wire protectors, by Beadalon®. 0.56mm inner diameter, open along the curve. Don't pinch closed — the horseshoe design allows for some slack on the cord to reduce stress. Similar to bullion, but sturdier, this preformed channel prevents fraying on stringing cable and cord. It is usually placed where you make a loop to attach the clasp. Best with 0.014" - 0.021" 19-strand to 49-strand Beadalon® and Soft Flex®.
I work in oxidized copper and ceramic clay so my wire guards need to be complementary to my other materials. These wire guards are ideal and give my bracelets and necklaces a complement I couldn't get when they were only around in silver or base metal.