I did some pieces a few years ago with regular cut pyrite (not pyritized anything). It had the strange property of not only darkening itself, but turning all the silver around it black!
G’day; Pyrite, whether copper, silver, iron or other metal based, is a
metal sulphide. In the presence of moisture and air, the sulphide
slowly oxidizes to sulphuric and sulphurous acid, particularly in
slightly damp conditions. (certain bacteria and skin compounds can
perform this reaction also) These acids act upon the sulphide to
generate tiny amounts of hydrogen sulphide gas, and it is the presence
of this gas which is causing discolouration and blackening of silver
by forming black silver sulphide around the pyrite. It won’t blacken
high carat gold because gold does not easily form compounds, even
sulphides. It might, though, blacken the copper in low carat gold
alloys I suggest that after work has been done on pyrite material the
job be well soaked in a solution of baking soda, then rinsed
thoroughly under a tap and completely dried. An application of a good
quality colourless lacer to the pyrite only, will help to stop future
blackening. Cheers, –
John Burgess; @John_Burgess2 of Mapua Nelson NZ