Oh hon - i know ... but can i just tell you, every day i look at my beautiful ring and just think how amazing it is that my boy chose that perfect diamond for me ... sure, it could have been tiffany but honestly, diamonds are diamonds and they are magical and beautiful and when you have it (no matter what size) you will love it.
For my 30th b'day my boy got me a tiffany heart ring with a teeny tiny diamond on it ... i just say 'well, ive already got a tiffany diamond' ... hee hee.
if i were you, and Tiffany matters more than 'size' then id do as one of the other girls suggested and get a celebration ring. Not all rings have to be single stones?

Do you know roughly how much budget he has?