Hm...sounds more like a trick question. It is true that normally guys only give girls jewelry if they really like a girl. Yes, you can quote me on that. For the newbies in the gifting scene, what type of jewelry should you get for your beloved?
It is the type of jewelry that does not require an intimate knowledge of a girl - like ring sizes, if they have ear piercing or not. Also, subconsciously where the jewelry sits on a girl's body is symbolic to a boyfriend's caress. So necklace in a way is more intimate than a bracelet. A bracelet is a nice first jewelry gift that does not carry too much of...say an "expectation."
Earrings are usually a pretty good choice. Earrings are usually easier for both the gifter and the receiver. A small pair of studs usually is a staple in a girl's jewelry box. Even if the girl does not usually wear simple studs, she can still wear it without affecting her style too much. And she can always find an occasion that calls for it. If you don't know what your girl likes to wear yet, stay with something metal or a simple gemstone set one (in a color that she likes) and don't go for anything too specific like pearls.
So the guy loves his girl but doesn't want to commit to the big M word yet. What should he give? Now is the time to get a necklace. A meaningful one is called for. Get one that would complement the girlfriend's style. It is the understanding of what she loves that makes the necklace a good gift, not how much you spend. But if you are hopelessly clueless about fashion, go for a nice one - if not a diamond, a precious gemstone necklace, or one that carries the meaning of love.
She is yours
It's time for the ring! Take a ring from her jewelry box and get it to a jeweler to find out what her size is. Or if that's not possible, try using a thin stripe of paper to measure the inside of her ring, input the length using this convertor to see what her size is. Do note that different fingers are different in size. To be safe, pick the middle finger. In winter if her fingers shrink in size in the cold temperature, she can move it to her ring finger and still be able to wear it easily. And it is easier to size up a ring then sizing down.
If this is your first ring for her, and that you don't plan on proposing anytime soon, stay away from a ring with diamond or white crystal/gemstone. See what necklaces and earrings she wears and pick a complimentary ring. Don't look for another ring she has and buy something too similar. But do use her rings as a style gauge. E.g., if she likes statement rings or simple everyday rings, with gemstone or not...etc.
Happy Valentine's Day!