What if you say/ do nothing and she moves on? I went to Uni with the most wonderful girl - she and I were great mates, spent a lot of time together. I liked her - I'm talking shedloads. We both had partners, I never told her. We're both married now and I don't see her much. When I saw her a few years back we were talking and I finally said "you know, I wish we'd gone out". Sure enough, so did she.
Point is mate, your lives will move on in one direction or the otheran and you'll both likely be happy long term with however it turns out.
Do you want to get to whatever age - maybe 21, maybe 30, maybe 40 - and sit there and think "I wonder if..."? I can't tell you what to do, but think about it, it's not a great feeling. Were I you, knowing what I know bow, I'd tell her how you feel, necklace or none. You can say you're aware of her situation, but you wanted to tell her for the reasons I've just mentioned. I understand your fear of rejection though!
Anyway, that's my two cents