Soldato
just bought one myself last week, this is everything I learned:
would avoid tiffany / brands - a diamond is something that is marked up pretty heavily anyway, let alone slapping a brand name ontop as well, you will loose a fair chunk of value to the shop straight away
wouldnt bother going above vvs1 for clarity, d-j for colour,
better to get a good balance of everything rather than just going for size, and above vs1 you wouldnt be able to see anything wrong anyway so not really worth paying the extra for
anything below 0.4c seemed pretty small, 2c seems massive
platinum is the best material for the band, although palladium is just as good really and cheaper, white gold seems to get knacked quickly
dont go any lower than "good" on the cut scale, it affects the sparkleynessness
dont buy one unless it comes with a GIA certificate
I went to brighton which has many many shops, so can get a fair idea of pricing / what you get for your money pretty easily
Would agree with pretty much all of that but would add that as far as clarity is concerned it depends entirely on the stone and the specific inclusion. Some can be really obvious sue to their colour and location others hardly visable at all you can also sometimes work around an inclusion by hiding it under one of the retention claws giving the appearance of a flawless diamond for much less price.
I'd also say as far as size is concerned don't get hung up on it as it really isn't that important a smaller excellent cut diamond will appear bigger than a larger poorly cut diamond in good light due to the radiance and sparkle. Also don't forget to take into account the lady in question if she has slim fingers large stones can look faintly ridiculous and they also have to be raised further from the finger to allow light to pass throgh the diamond which can make them awkward in the real world.