From here:
https://www.quora.com/What-will-the-iPhone-8-be-made-of/answer/Brian-Roemmele?srid=Pi3
The Landmark Zirconia Ceramics iPhone, MacBook And Apple Watch Patent
On August 3rd, 2015 Apple presented the landmark patent for the future direction of Apple products to the USTPO. Innocently titled: “CO-MOLDED CERAMIC AND POLYMER STRUCTURE”. On September 8, 2016 one day after the Apple event that announced the iPhone 7 and the Apple Watch Series 2 the patent was made public. Of particular interest is Apple Watch Edition Series 2. Although I anticipated a ceramic Watch but this time Apple used Zirconia + Alunima. The patent was embargoed since August, 2015 by the USPTO to one day after the event so as not to telegraph a future product shift to competitors.
The author postulates that the Apple Watch Series 2 Ceramic is a harbinger of other ceramic-based devices. Exciting stuff.