How much should I spend?
It's up to you and your bank account. There's no need to go into debt or adopt a ramen-noodle diet for the foreseeable future to buy an engagement ring you really can't afford. Ignore that "rule" that's been around for decades about how you're supposed to spend three months' salary on the ring—spend what you feel comfortable with, balancing size with quality and keeping in mind that, unless you've already got a healthy bank account or generous parents, you have a wedding and honeymoon you'll most likely be springing for too.
Is the price negotiable?
Sometimes, so it's worth trying. Research comparable diamonds (size, color, etc.) online and at local retailers, and have those quotes on hand while negotiating. You may have a better shot at a discount if you mention you're willing to pay in cash.
Do I have to buy a diamond?
If your girl has a thing for other gems, like rubies, emeralds, or sapphires, or shuns convention, consider another kind of stone, unless she's told you she has her heart set on a diamond.
What style should I get? I don't want to get something she hates.
Look at the jewelry she wears a lot. Is it simple and understated, or glitzy and oversized? Gold or silver? Is she the type who will thrust out her ringed hand every chance she gets or keep it on the down low? Give these details to your jeweler so he can make suggestions that fit her style and personality.
Is there a way to make a small stone look bigger?
Yes, there is, you sly guy! Get a diamond that's considered well cut—its facets will interact nicely with light, creating sparkle, which will make the stone look bigger. Certain shapes like round, oval, and marquis set on a thin band also trick the eye into making a diamond look bigger, as does a ring with a center diamond surrounded by colored stones. Also consider a platinum band, whose white metal will reflect light and act like a mirror.
Without asking her, how do I figure out the ring size?
Be sneaky and grab a ring she only wears occasionally from her jewelry box or trace one of her rings on paper when she's not around.
Do I really have to study the four Cs? What does that even mean?
While you don't have to sign up for classes at Diamonds University (if there were such a thing!), you should have a basic understanding of the four Cs (cut, carat, clarity and color). When the jewelry asks how many carats you're interested in, it should be clear you know he's not talking about vegetables!