I want to propose to my girlfriend

[Picasso]

After 3 years together, I finally decided to propose to my girlfriend. Got to the point where I know I want this to be a "till death do us part" kind of thing. What should I even be looking at for the ring? Price genuinely doesn't matter, I just want something that'll actually take her breath away when she sees it.

[Bermudo]

An expensive ring doesn't automatically mean a beautiful one, the cut and shape of the stone matter a lot more for how it actually catches light on the hand. Oval cuts are especially popular right now, they make the finger look longer and the stone tends to look bigger than its actual carat weight compared to a round cut. Sizing is trickier when it's a surprise, but borrowing one of her existing rings for a day or asking a close friend who's in on it usually gets close enough.

[Cabanas]

The setting matters just as much as the stone itself, a thin band keeps all the attention on the center stone, while a setting with side stones adds more overall sparkle. You can look at a ring like this one, for example: http://pierrejewellery.com/katalog/oval-diamond-engagement-ring-5329 . The oval diamond in the center gives exactly that breath-taking effect you're going for. The side stones also add extra light play, so the ring reads as more expensive than a single stone in a plain setting would, and most jewelers can still resize it slightly after the fact if needed.