New York diamond district

I will be travelling to New York in a couple on months to purchase a diamond for a lost stone on a ring.
Has anybody any advice on which stores in the diamond district offer best value and service.
What are the tax implications in buying a stone in New York (do I pay tax on it)?
Any other advice anybody could give will help me out a lot. Don't want to get ripped off over there or purchase a poor quality stone.
I will be looking for a certified stone, but how do I know the cert is real?

Thanks for all the help!

You may find some useful info on this key post on Engagement Rings.