Sindarin 

rustui

adjective. copper, copper, *of copper, like copper

An adjectival form of rust “copper” in notes from the late 1960s (VT41/10).

Conceptual Development: The Gnomish Lexicon of the 1910s had G. tambin “of copper” and G. tambrin “like copper”, both based on G. tăm “copper” (GL/69).