This is a so-called Clicker ring with a small hinge located in one part of the segment, which makes it very easy to open and close!
Steel Blackline® is 316L Steel covered with a thin plasma film (PVD-Coating). The thin film is very strong and does not fade. The technology has been used in the medical industry since the late 80's and is used, among other things, for pacemakers and orthopedic implants to provide an extra durable surface.