I have 2 x the L version ordered 4 cm diameter, the brand of stainless steel is extremely robust, which also falls literally according to "significant". The engraving is easy to read (name of my dog + 2 phone numbers were engraved). Stainless steel. The supplied loop is of good quality. I would order these dog tags Red Dingo definitely be back.
Addendum August 2013: For our Medium Schäfermix bitch I ordered a brand of size M, on her name (5 letters) and 3 (!!!) telephone numbers were engraved legible.