I purchased a diamond 10 years ago from Fidelity Diamond in Paramus NJ.
http://www.fidelitydiamond.com/
I had a very good experience. The sales person took their time with me --almost 2 hours inspecting each diamond and I finally picked one.
I purchased one a little over a carat for about $4,600. Got it appraised the next week in the city by a certified diamond appraiser for well over the purchase price- it might have been around 7,000??
Like I said it was 10 years ago but I had a great experience.
Good Luck! You will make someone very happy with a big rock like that!