Due to Fox having the rights to X-Men, there are few restrictions to what can be mentioned and alluded to in the Marvel Cinematic Universe (e,g., 'mutants'). Does this restriction stretch to adamantium? I understand that, in the comics, some incarnations of Ultron were made of the substance.

I'm not asking, in the upcoming movie, Avengers: Age of Ultron, if Ultron will be made of adamantium. I would simply like to know if it is/was ever an option?