Apple once considering offering the Apple Watch Series 5 with a black ceramic casing. Photos of a prototype case for the watch were shared on Twitter by leaker Mr. White.
Apple only offered the ceramic Apple Watch Series 5 in white with a starting price of $1,249. A black ceramic Apple Watch would have looked even more stunning, but it is possible that Apple faced some production or reliability issues with this option which is why it decided against it.
From the photos, the black finish also looks similar to the Space Black Stainless Steel casing of the Apple Watch Series 5.
https://twitter.com/laobaiTD/status/1405132162337107976
Apple no longer offers ceramic casing as an option for the Apple Watch Series 6. It is likely that the ceramic casing, despite its scratch-resistant properties and lustrous finish, did not attract many buyers due to its relatively high price tag. The company currently only offers the Apple Watch Series 6 with a titanium casing.
Apple tends to try different materials for product designs and cases, but not all of them pass through its testing. Some materials are also rejected because they are difficult to procure or are too expensive to work with.