If she doesn't believe you love her, don't try to win her over with presents. Show her you love her by the person you are. I think a present to prove your love to someone can seem like you don't have enough substance to your own person so you have to resort to presents to supplement the goodness of yourself. I'm finding it quite hard to say in words what I mean so I hope this makes sense.
If it's not quite as deep as that, then make the present a personal one. Personally, I hate jewellery, especially jewellery I haven't chosen because there are very few items of jewellery I have ever really liked.
Think carefully about her likes and dislikes. Give her something creative and personal to her, something that she would love and it would mean something to her, but no one else would think it so special. For example she might have mentioned something she likes/wants. Don't get her something you could have got for anyone, ie standard 'presenty' things like jewellery, chocolates, gadgets etc.
For my best friends' birthdays I got them personalised t shirts; I designed a print (saying a personal joke and a little miss picture) then took it to a shop and they put it onto a t shirt for me. You can personalise more or less ANYTHING like that, mugs, mouse mats, clothes etc. I'd be so touched if someone gave me something personalised like that!
If you do decide to do something like that then get something on it that relates to BOTH of you, that way it will remind her of you every time she looks at it.
Hope she likes whatever you get her!