rust is to iron as ___ is to silver

Answer 1

Not much to go on but I would guess perhaps silver oxide?
Rust is iron oxide.

Answer 2

I don't know if this is silver oxide -- but tarnish could also fit in that blank.

Answer 3

'patina' is the word for the discolouration on silver.

I haven't seen this question for over 30 years - definitely recycling!