believe me, buying jewelry from the NY Diamond District is the BEST way to go. you will get amazing deals there and they're very honest.
i grew up being schlepped there as a kid by my mom who was obsessed with buying most of her jewelry there. my parents are both from new york city and it's something that is very well known and very trustworthy. you're not going to pay the added fees that you will at a place like tiffany & co., cartier, et al, because of their lower-cost overhead. and you're not paying for the fancy boxes, etc. just good old fashioned jewelry.
my mom bought her gorgeous 3 carat pear diamond platinum ring from the diamond district back in 1974 (which was also an estate piece)—and now it is mine. i was only 9 years old and i remember going with her when she bought it.
just take a walk up and down and go to every store and see what catches your eye. you'll be amazed and what you can get as opposed to a mall-type jewelry store.
good luck and have fun!