By: Ed. W : July 2nd, 2013-06:38
From the GS website:
"Every surface of each case and bracelet is polished by the experienced craftsmen. The mirror finish on the case side surface has no distortion. The process is known as Zaratsu or blade polishing and is made by carefully applying a rotating tin plate against the case at a very precise angle. This process is entrusted to only the most skillful and experienced craftsmen."
basically, it's tin polishing for the case. The same procedure (polishing steel on tin plates with ultra fine diamond paste) is done by brands like Glashutte Original for their swan necks and other brands for steel components to attain what is known as black polishing, where the surface is so highly polished it looks black from certain angles. This is why the GS case is unmatched in my humble opinion.