I could easily see that happening with the old back plates Apple used, but with the new ceramic ones, I don't see how it would get scratched that bad, unless you were intentionally doing it.
If you look at some electron photography images of the DLC coat, you'll see that it looks like a mat of tiny little nodules - those would be super-hard diamond-like nodules btw - that probably are fairly abrasive due to their sandpaper-like appearance. So the zirconia ceramic being softer (relatively speaking...) than diamond, the potential for scratching is heightened.
...Possibly. Not a materials engineer. Just speculating. *shrug*
The circle is metal, as that's what's used for the ECG to complete the circuit
The ECG electrodes are a thin (likely vapor deposited) metal-nitride film on top of the sapphire center dome. It's not solid metal, perhaps for weight and/or manufacturing reasons.
Btw, I've been wondering how wear resistant the electrodes are, especially with attaching/detaching the charger puck like a thousand times over the course of a few years. Hopefully Apple has tested their choice of materials carefully and deemed it of sufficient durability...
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Makes one wonder why they should pay more for quality that is not really there.
It's not a case of (missing) quality; it's fundamental physics.
Apple could have used a softer coat on the space black watches, but then it would have been less durable, and the stainless metal would eventually wear through and the whole watch would look really cheap and bad.