youll probably be fine. Almost all latex garter belts/girdles/etc use silver metal stocking hooks in them, and its never been a problem for me on black latex.
Colors you really have to be careful of are the lighter ones and transparent ones.
You can also spray the necklace with a clear varnish
found in any walmart, home depot, lowes, michaels/joanns craft store, etc etc
Or paint it with clear top coat nail polish
that will put a barrier between the latex and metal