Absolutely agree that finding out about and being sensitive to her tastes is fundamental. Many, maybe even most (from what I hear) women will be very picky about diamonds, in fact I've read on these very boards that stone counts above all else and setting is secondary. However, a lot of women still prefer something else.
My own ring is fancy-colored sapphire with a hand-carved artisan-made platinum setting, it's unique and I love it, but it's still more classic than much of my other jewelry, so I can wear it throughout the decades as I age and my tastes change. As a stone a sapphire, even a fancy-colored one, is less expensive than a diamond, although actually more rare, and it's very nearly as hard and impossible to damage...the platinum setting brought the price back up somewhat but that was my partner's choice, I was fine with palladium, anything but gold.
My partner had an advantage in that we had already talked about getting married and so I was able to tell him some things, such as that I did NOT want a diamond unless it was recycled out of my grandmother's old jewelry or something...I just didn't want to worry about blood diamonds plus I hate the way the diamond industry runs its campaigns. We still ended up shopping for it together because he knows I'm picky about my jewelry

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I totally understand wanting to surprise her with a ring at hand, but unless you are confident in her jewelry taste, it might be a good idea to surprise her with a temporary ring...most local jewelry stores will lend you a temp ring if you give them a down payment as a guarantee that you won't run off with it and will return to shop for the real ring. Just make sure that it's a store that has a wide variety of styles or is willing/able to custom-make rings so she has a lot of choice available, because once you give the down payment you're kind of locked in there.
If you still prefer the idea of surprising her with THE ring best, talk to her people first. Ask her best girlfriends, maybe her sister...if there's one you trust to keep their mouth shut and to know your girlfriend's tastes ask them to go ring-shopping with you, or at least to send you some pictures of examples online.
IMHO, it is always better to shop for a purchase like this in-person rather than on a website...and I am the queen of online shopping because I hate malls and standing in lines.