Birthday jewelry gift for my girlfriend?

OK, so its her first bday with me, about 3 months dating and I love her but havent told her yet.. I dont have tons of money after the dinner and all the other stuff I'm paying for, but would like to get her maybe a silver heart on a chain and profess my love to her, maybe with an inscription. i live in NY near the city, any suggestions of what to get specifically in terms of a nice little heart necklace or something romantic to profess my love with that is not much more than $100 ?