That's easy, just put it in a sealed container with some burning sulphur (i.e. throw in some lit matches) and it'll turn as black as the ace of spades.
. Ahhhhh of course: 'Tarnish' = Silver Sulfate It's basic chemistry 101.....but yeah: What about 'Rainbow Toning'?
Artificial rainbow toning is achieved by electrolysis in a sulphate bath. Different voltages produce different thicknesses of 'tarnish'. It's light being refracted as its reflected through sulphide layer. Different thicknesses of tarnish produce different colours.