I highly recommend Whiteflash!

Whiteflash provides outstanding service both online and in-person. Their website has an immense amount of data regarding the diamonds that you don't get from most other online vendors. For example, you get the ASET and IdealScope images for the diamonds. Furthermore, they actually possess the diamonds they sell, so you know the data is valid. It was a treat to work with them to pick out the ring and finalize the decision on the diamond. They made the entire process a breeze. They thought of everything, including shipping in a manner such that the surprise could not be ruined. I highly recommend Whiteflash!