Neutral colours for emeralds and rubies. Both could work well with dark colours or light I think. I would stay away from glittery polish though maybe...but I could change my mind on that if I saw one I liked. For a big blue sapphire, I think anything goes; just like a great pear of jeans.
I was actually just thinking about this tonight as I did my nails for my ruby that should be coming this week. I picked a gun metal silvery shade.
I'm having a sign made so I can follow Jcent around and go "yeah, that!"
Anyway, I do neutrals and opalescents with my ruby. I have done glittery polish and thought it looked good, but not everyone agreed! Really though, the ruby is so bold that a light pink, nude, or sheer polish works best for me-but that's partly because she's in yellow gold too. I've done red before, but haven't loved it. I do have one OPI shade that looks okay....but only if there's no other color going on.
Gunmetal is an EXCELLENT choice for a ruby though! Jcent, is it in white gold? That will look AMAZING.
Back when I had a sapphire, it went with pretty much anything. I will say that I personally kept it pretty minimal though because I liked that sapphire being center stage and not competing at all. But I'm a minimalist at heart anyway so that plays into it. That and the fact that my fingers are short and stumpy so the polish is very close in proximity to the stone!