I've had plenty of friends go through engagments... here's my 0.02...
definitely take one of her friends, this will help
-make sure you get the right color (gold/silver). Also it'll help you get an idea of good ideas over bad ideas ... (e.g. good idea: princess cut on white gold, bad idea: pear shaped on yellow gold)
- try to figure out if she would like a stone set up high, or something more inlaid. Again maybe a friend could find out.
- Is she traditional (solitaire)? more modern (three stone)? glitzy (throw in some channel set stones)?
If you can get these answers, and you stay in the right ballpark of 'her,' then it'll be hard to miss ... the ring is coming from you, so it isn't 100 percent her anyway.
As a side note, sometimes I don't want a diamond in my ring either. (1) There's something elegant about a band - (example, Mr. And Mrs. Smith - Angelina Jolie's character just had a band), (2) the diamond industry is sketchy at best, and (3)the only reason I think I want a diamond is to make my family happy - I don't wear jewellery all that much.
If the ring came from an gradparent or a family member, then I also wouldn't care what it was.