My Girlfriend is allergic to some metals?

Im going to get a Promise ring for my Girlfriend. She already knows im getting her one but I know that when she wears "fake metal" her skin turns green, and I dont want that to happen to her :( is sterling silver a "fake metal"? I really want to know that because I have to choose real stone or real metal but I found one with Sterling silver and real stone so...I want to get her that one :) but I dont want her to not be able to wear it :( Please help fellow Yahooligans?