I found the following on an internet website about jewelry care:
All jewelry made from platinum, gold or silver tarnishes over time
from the process of oxidation. A film or thin layer of
discoloration develops on the exterior of these metals.
This created some discussion in our store with regard to whether or
not platinum jewelry can oxidize over time. My only concrete
experience has been with customers who bought platinum jewelry and
after the purchase, due to ill health and changed medication,
complained of discoloration. One specific customer purchased a large
platinum ring after years of buying gold from me, and thereafter was
required to take high doses of lithium. She came in highly upset
about discoloration to her rings. I don't know that this was an
oxidation issue, but we determined it was a chemical reaction due to
the PH change of her skin. She had worn the ring for half a year
with no problems.
Having found the above quote I did a search of the internet for
further and found other comments about heirloom jewelry
made of platinum "darkening to a dull gray" over time.
I'd like to hear some thoughts on this since platinum is primarily
recognized as being oxidation resistant.