If you're not going shopping together (click here to see if that's a good idea), then you need to do more than just plunk down cash... you need to guesstimate her ring size. You need to figure out the metal. (Probably platinum.) You need to figure out her preferred style, setting, and stone shape. You need a plan.
This isn't an exact science. And there's no perfect solution. These ideas, however, will at least get you in the right ballpark. Good luck.
Snoop around in her jewelry drawer. Find another ring of hers, steal it, and then take it to a jeweler. As long as it's the right finger, the jeweler can use this to get her size.
This one is so obvious it gets overlooked. You can just put one of her rings on your finger, slide it up as far as it will go--ideally, your fingers are fatter than hers--and then mark the spot (remember whether you're marking the side closest or farthest to your fingertip). The jeweler can use this to measure her size.
Full disclaimer: we've heard this mentioned by friends—and it's a popular recommendation with other experts—but we have not actually tried it ourselves. So use at your own risk. Basically, you grab a bar of soap, fetch her ring, and press the ring against the soap to make an impression.
The styling is more complicated. There are a number of key decisions to make: the shape of the diamond, the type of setting, and the style of setting.
Something like 75% of the diamonds sold are round, but there are other shapes that are popular and, depending on your budget, might be a good fit. The safest choice, however, is to go with round. The basic setting choices are solitaire, three stone settings, and sidestone settings. You will also have to decide what metal to buy—yellow gold, white gold, platinum, etc.... Finally, the setting style: classic, modern, or antique-looks. Frankly, Blue Nile does a great job explaining the shape and setting choices.
It won't work. We've seen other articles that advispe things like, "Pretend that you want to buy a ring for your sister, and then ask what kind of rings she likes to wear." This is about as plausible as Hillary Clinton's tales of "sniper fire" in Bosnia—it simply won't hold up. She'll see through it.
At the very least, you should get an idea about the kind of metal she wants. Does she always wear silver or white gold? Then it's probably best to avoid yellow gold (platinum is the safest option, anyway, see below.) Get a sense for her style. Our tip: actually take some digital photographs of her jewelry and show it to the merchant—this way you won't awkwardly fumble at the store and say, "Um...she likes shiny stuff, I guess."
This could be better in theory than reality. The idea is that you talk to her BFF, swear her to secrecy, and get the BFF to find out her ring size, preferred settings, etc. Only three problems with this: 1) Who knows how trustworthy this friend is? Are you sure she can keep a secret? The downside is astronomical. 2) Who knows if her friend actually has any frickin' clue what kind of ring your girlfriend wants. She can probably help on the size, yes, but the rest is just guesswork. 3) More philosophically, do you really want someone else to know that you're getting engaged before your girlfriend? In an ideal world, she should be the first to know.
Remember that bag of sand Indy uses? At the beginning of Raiders, he swaps the golden idol for a bag of sand—keeping the weight balanced—and avoids a lethal trap. You can do this with the engagement ring. When you propose, you can give her a "stand-in" ring--ideally a family heirloom--that you later trade for the real thing. This lets you propose without some lame "IOU" but you let her give input into what ring she gets.
As far as settings and ornamentation, it's better to err on the side of simple and conservative. Just focus on getting the best diamond in your budget. Once you propose, tell her that you're fine with changing any aspect of the band or setting. A friend of mine tried to channel his inner-jeweler and was "inspired by California poppies"—whatever the hell this means—and wanted to get her an orange stone. Debacle. Just play it safe, get a good diamond, and give her carte blanche to reset the sucker.
No matter what you choose, there's a good chance that your girlfriend/fiancé might want some type of modification. Make sure that your exchange policy will let you swap out your ring for a change or (gulp) upgrade. This way, when you give her the ring, you can let her know that it's not final-final-final... she can make any changes she'd like and you won't be offended.
Some parts of ring-buying are tricky. This one isn't. It's the easiest call you will make. Platinum makes the most sense for about 3,000 reasons. 5 quick ones: 1) She probably wants it--80% of women prefer platinum. 2) It holds its value better over time. 3) It's more durable than gold. If it gets scratched, no metal is actually lost, it's just displaced. 4) It's higher quality. 5) It holds the center rock--the absurdly-expensive diamond--more securely, so it better protects your investment.