The interior has been aluminum, but the band around the outside has been stainless steel.
> Ceramic Shield front, Textured matte glass back and stainless steel design
https://support.apple.com/kb/SP875?viewlocale=en_US&locale=e...
> Ceramic Shield front, Textured matte glass back and stainless steel design
https://support.apple.com/kb/SP875?viewlocale=en_US&locale=e...
> Ceramic Shield front, Glass back and aluminum (sic) design