What is the black accent in the digital crown ...

It serves no purpose and is cosmetic only..

 

The Diamond like Carbon coating is the material surrounding the casing and crown.

 

Isn't it ceramic? On both SS and SBSS

 

I've never found a true answer, but I posted this a while back, which suggests that whatever it is it's pretty durable. Possibly ceramic, sapphire or zirconia.

"Found this, supposedly from their more recent digital crown patent according to patentlyapple.com (patent heading reads "COMPRESSIBLE SEAL FOR ROTATABLE AND TRANSLATABLE INPUT MECHANISMS":

"FIG. 3 is a partial cross-section view of the electronic device 102 taken along line A-A in FIG. 1. As illustrated, the input mechanism assembly 110 may include a cap 303 (such as zirconia, sapphire, and so on) fitted into an aperture of a manipulation structure 148 (such as a knob that may be made of aluminum, gold, or other material with a variety of surface finishes such as matte, polished, and so on). The cap 303 may be fitted into the manipulation structure 148 via an adhesive mechanism 278 such as heat activated film, pressure sensitive adhesive, and so on."

Note the mention of zirconia and sapphire, suggesting that even though here it isn't specified, they clearly had in mind something wear-resistant."

 

This is what it looks like to me too. It looks the same as the back sensor

 

I was wondering the forum member who was asking this a while back. I think your answer is the closet it can get. Defintely very technical.

 

It's the same colour as the HR sensor/cover so I would have a guess at zirconia!

 

True. That makes the most sense. And I saw a picture from some internal Apple repair guide that had a picture of a broken cap insert that was cracked in half (as opposed to The type of damage you'd see for a piece of sapphire). Looked like ceramic.