This has intrigued me for hours Juscha, as I can see what you mean about the lack of tarnish in most areas, but there is tarnish in some inaccessible places. I have the feeling that all the jewels have been cleaned to within an inch of their lives! Perhaps that is why yellow gold is showing through in places.

Yet, if the jewels had been rhodium plated, I would expect the plating to wear off more quickly in other areas.

I think this is one area where we need expert advice. The gold we can see suggests that, underneath the white looking metal, is yellow gold.

You have presented a little mystery which has captured my imagination.