One thing to keep in mind when buying a diamond is that no two are alike, think crystalline snowflakes glistening on a sunny day. Diamonds are complex, learn what you have time for and so your up on the lingo and hit the streets. If you can't see the actual diamond for yourself, you should choose to use a person you can communicate directly with, who based on their experience can select stones for you based on your budget.
In addition to the fab 4 C's of diamond grading to consider are table and depth percentages and the part she admires the most, the fire or sparkle. The Four C's refer to quality grading, and is a system developed by the GIA (Gem Institute of America) as a fair and consistent way to grade diamonds.
Read about the 4 C's on the GIA's web site.