Have you tried titanium posts? They are, IMHO, in another whole
league than the "hypoallergenic" junk out there. It's an elemental
metal, not an alloy, and it is quite inert. You can always have some
18K posts welded on, but there you have an alloy again. If you insist
on doing it yourself, you'd want to remove the amethysts and coat the
diamonds before soldering. But aren't you sort of a beginner
solderer? Maybe let someone else do it.
I can send you a pair of titanium earwires to try out on your ears,
if you like.