Before we got engaged, we both knew that marriage was where we were headed, so we went to look at rings together. We talked with jewelers, got sized, and most importantly he got an idea of what I liked: cut, color, setting, etc. Soooo, I didn't pick my particular ring out per se, however, he took a little bit of everything I liked and custom made my ring

. Had it been in the showroom, this is the one I would have picked.
He didn't propose until about two months later though. It was great because by looking at rings together, I knew he was serious about marriage, but at the same time, I had NO clue how soon the engagement would happen.
Now I was excited about the prospect of getting engaged, yet I didn't tear myself apart like some girls do waiting for the question to be popped. I knew it would happen eventually, so I didn't stress over it after looking at rings. I would suggest you do the same with her,
UNLESS she is the type that is going to drive herself nuts waiting for you to ask her. Girls like to look at jewelry, and at the same time, like that you care about her interests. It also depends on how well you know her. If you know her tastes really well, I am sure you'll do a fine job of picking one out yourself. My beau and I looked together because I like less traditional things and he is more classic; plus we wanted eachother to like the rings too...a happy medium if you will...I would have hated for him to buy me something he couldn't stand! There is nothing wrong with a surprise engagement ring either, I don't think any girl would complain about the ring she is offered! There are some "shallow" girls that are soooooooo concerned over "how big my diamond/gem is!" but most girls could care less.
To be honest, girls just want to get "engaged" and really aren't picky about the ring!
I wouldn't stress over this...You know her best and whatever you decide, she'll say yes!