I once placed a small sterling casting in the cavity of a halved,
quartz crystal lined geode for display purposes. When I examined it
several weeks later the once shiny casting was completely blackened
and remains so to this day, despite repeated pickling and buffing in
a vain attempt to restore its original lustrous finish.
I am unsure of the mineralogical classification of the geode or of
its geographical origin but it has a faint petroleum odor, which I
failed to notice when I first assembled the display.A0 My guess is
that it contains a sulfur compound that is more powerfully reactive
with silver than is liver of sulfur.
Someday, when I have more time, I intend to experiment further; as
the patina that I unintentionally produced is amazingly durable.