Right after getting my Series 3 last year, I accidentally brushed against the wall in my basement (concrete). It scratched the DLC.
I went nearly a whole year with a scuffed up Space Black Stainless Steel watch. Before getting it replaced with Apple Care, I wanted to see how durable it was.
I found out that the "Diamond Like Carbon" is a
lot softer than the sapphire display.
Regular sandpaper will take the black DLC right off. Scraping the watch against my floor tiles took the DLC off like it was nothing, leaving awful silver shining through.
I have gallery of some pictures of my old Series 3:
https://imgur.com/a/LxlY3nv
Knowing now how delicate (and non-reparable) the black DLC is, I'm tempted to get the silver SS next time, as I know it can be buffed and polished if scratched.