you can use the ''pricescope your diamond'' tool at the top of this page and select ''cut quality search'' to find a listing of top quality loose diamonds.
Depends on how much self education and work you want to put into it:
If you just want to go two several vendors who sell nothing but top notch diamonds from a number of perspectives - and buy now: I''d go to NiceIce, Good Old Gold, and Winfields (Wink Jones) which are my personal favorites.
Whiteflash is also a very good company.
If you want to spend a week or so learning about diamonds and then do some searching and sorting - there are a number of second tier companies that sell a broad range of diamond qualities; but allow you to relatively easily find good ones.
If you want a "good" diamond - knowing what a really good diamond is - for the least amount of money and are willing to spend several months learning and searching; then welcome to pricescope, buy the Idealscope, light table, and other tools; and may the journey begine. In this case though - a number of people end up dealing with the companies listed above even if they arn''t the absolutly cheapest due to the other services that they provide.
I note that others may have a different list of "personal best" and may some other companies.
I''m very biased (because I got my diamond yesterday) but I would go with WhiteFlash. We''ve had a terrific time working with Bob over there. He was amazing and patient with us. And my ring absolutely is better than I expected.
You''ll get great testimonials for many vendors on this site. There are some fine ones which have already been named. I have had the pleasure of talking with most of them! The key is which one has the diamond you are looking for! We ended up buying from Good Old Gold. I am biased, too, because Jonathan is just wonderful. He is a gemologist and has such a passion for what he does. No one does any more testing and analysis of diamonds as he does. If you want assurance of quality, you have it there.
Look at the 4C''s section of the Good Old Gold site for the best pictures on the web regarding diamond color, clarity, etc.