Green patina that forms on copper, brass and bronze — diver's rig (anag) (9)

I believe the answer is:

verdigris

(Other definitions for verdigris that I've seen before include "Greenish-blue coat on copper" , "Greenish blue coating on copper" , "Green deposit" , "Patina on copper" , "Green patina" .)

I've seen this clue in The Guardian.

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