I got a fab white gold & diamond eternity ring in Leon Diamond, 74 W 47th Street (212) 302 7327. They altered it within 30 mins for me as I wanted a comfort fit to the back of the band - very very reasonable
Head for Haniken's also on W 47th St. They are excellent for both diamonds and wedding rings etc. A good friend who is a stewardess with Aer Lingus goes there all the time and says the rest of the airline staff get their jewellery there as well. I bought an engagement ring there earlier this year and Mrs som was v happy with it. Massive savings to be made compared to Dublin Jewellers