As a chemist, perhaps I could offer a bit more advice on removing mercury. Mercury evaporates a little below 400C and if you put it in a domestic oven for a while you may be able to get rid of it that way - but the opals probably won't like it.
The opals DEFINATELY won’t like it. Plus, if I might, it seems of
questionable wisdom to suggest evaporating mercury in an oven also
used for domestic food production, and most likely not equipped to
vent the produced mercury fumes outside. Mercury which evaporated in
your domestic oven (few of which actually reach 400C) would possibly
then recondense in cooler, outer parts of the oven casing, causing the
risk of food contamination as well as general low level atmospheric
contamination of the home interior. As you’re a chemist, I shouldn’t
have to remind you that mercury is not benign. And even low levels
of mercury fumes should be avoided. Even more, I should think, it
should be avoided in food preparation equipment…
Unsetting the stones is probably the safest way. However Mercury is soluble in nitric acid and if your gold is a relatively high grade you could try immersing the affected areas in, say 10% nitric acid -
10% nitric won’t have much damaging effect on golds of 14K or over.
It may tarnish them, fixed by a quick polishing afterwards. 10K will
be attacked however. Opals, on the other hand, won’t be damaged by
dilute nitric. Some stones might be however… Acid removal of
mercury works, but must be done almost immediatly (within hours) after
exposure to the gold, since the mercury rather quickly starts to
penetrate the gold surface, and the acid will not remove it once it’s
penetrated into the gold.
The mercury will wear off/evaporate off eventually (weeks I think) if you leave it alone. You will have to repolish as the amalgam attachs the gold.
It won’t completely wear off or evaporate. A sufficient quantity of
the mercury is able to penetrate the crystal structure of gold alloys,
forming an amalgam . This can penetrate surprisingly deeply into the
gold. It’s not just a surface effect, and it can happen fairly
quickly. Just leaving the item alone will let the appearance improve
over a few weeks, as the surface film of mercury dissipates. Partly
it evaporates, but much of it just diffuses further into the gold.
Repolishing the surface might look OK, but it does not solve the
problem. And the problem is that mercury contamination of the golds
leads to quite brittle behavior. Mercury that has penetrated into the
gold will lead to cracking and failure of the jewelry item, especially
things like prongs or thinner sections. It really MUST be properly
and completely removed if the jewelry item is to remain properly