Get a feel for her favourite stone/metal. If she wears a lot of gold, she won't appreciate silver, vice versa. If she likes diamonds/CZ/etc, she probably won't like rubies/sapphires etc.

Get her friend to ask some questions about style, you'll be all set.

Hey guys,

Going to pop the question to my girlfriend while over in Europe soon. Starting to look into rings and I have to be honest, I can't tell the difference between one of the $2 rings at the markets and the $20,000 rocks, I just lack that part of the brain. They all look the same to me.

Does anyone have any experience here on major do's and don'ts. I'm going to talk to one of her friends and probably go with her to have a look but I need all the help I can get.