Wired runs down the properties of the ceramic that the new $1250 Apple Watch Edition is made out of:
Watchmakers use a ceramic material with very fine pores. It’s called zirconia (zirconium oxide, for the chemists reading), and it’s simultaneously hard and resistant to cracking. It also resists changes in temperature and moisture, which is why surgeons often use it in hip-replacement prosthetics. To bolster its strength (and achieve that bright, white color) Apple added alumina, another ceramic. The result is a material that’s pretty much un-scratchable, and, under most circumstances, unbreakable.
I’ve banged around my aluminum Apple Watch Sport for the last year and change and have never seemed to do any permanent damage to it. (Though I don’t wear it while playing sports—mainly because I’m worried about hitting someone else with it by accident.) But as the article also reminds us,
all the existing stainless steel Apple Watches use ceramic on the backs, so the material isn’t exactly new.