According to Mr White on Twitter, he shared images on what appeared to be an Apple Watch Series 5 in a black ceramic finish. This Twitter user is also famous for sharing images of unreleased Apple products, including prototypes.
The Apple Watch Edition started out as a true luxurious lineup of smartwatches with 18K-gold models starting at $10,000. The Series 2 of the Edition lineup replaced those expensive models with white ceramic cases. Those started at a much lesser price point at $1249. The Series 3 introduced an addition gray ceramic finish before Apple stopped selling the Edition lineup with the Series 4. Apple did bring back the Edition lineup with the new titanium cases, and only the white ceramic case returned. The titanium cases came at a much lower starting price point at $799. With the Series 6, Apple discontinued the ceramic model altogether, leaving the titanium cases in the lineup.
While Apple appeared to have had plans to release this finish for the Apple Watch Series 5, it didn’t make it to the public release most likely because the finish looks too similar to the Space Black stainless steel case.