I bought a ring earlier this year. Spent $10,000 (I had it later appraised at about $16,000)
First off, biggest waste of money (besides cookware!). But it is for the girl so I think you should sleuth it up to see what kind of diamond she wants. Of course they want colorless and flawless and big, but see what kind of shape she wants (round, emerald, princess, heart, etc.).
Settings are tough as well, if she is particular, you may want to get a simple one and have her design it later. If you have good taste, go for it.