It's a shame that Fusion 365 is such a cloud-& community based software but does not yet support a community build library for materials & appearances. I would love to see this option on top of the current three options: "Fusion | My ... | Favorites"
The 4th option would be " Public (cloud)" and it would be possible to submit your own custom materials - probably moderated by either community mods or somebody at Autodesk.