Something worth discussing wrote:
NYC diamond district
Yes but don't just stroll into any of them. check out Jangmi Jewelery in the NYC diamond district. There are also a few other good ones
Only buy GIA Certified. The other certifications/ratings are not as solid. Do some research
I would consider a round diamond possibly "hearts and arrows" cut (this just refers to the symmetry of the diamond... You don't need a massive rock to really catch and reflect light beautifully)
you need to see the diamond in person, so I wouldn't recommend Blue Nile, whiteflash, James Allen etc... MAYBE Brian Gavin, but way better to go to Jangmi in person. There are probably very good places in Chicago, LA, SF too if you are closer. But New York is better, just research beforehand because there is also a lot of BS in the NYC diamond district.
You pick the diamond (maybe just put it on a simple setting- solitaire or whatever), then you go back together and she can pick the real setting after you propose