It depends what she likes, and how serious you are.
A diamond is classic - unless she has expressed a dislike for the stone, or refuses to wear classic jewelry, I would go for that.
You should ask a friend of her about what she would like. What you should make sure of, if that it is a special piece that she will enjoy wearing for years, and that can be a symbol of your will to be together forever (so, no fashion/custom stuff...)
There are also other stones - look into sapphires, rubies, emerald - sapphires are quite popular, as blue is the color of faithfulness, Emeralds - green is the color of hope, rubies, red, the color of love... :) or also, opales,pearls can be absolutely beautiful (although pearls are told to be "unlucky" because they look like tear drops). And how about Aiguemarine - beautiful color, beautiful stone.
A reputable jeweler would also be able to assist you in designing a one-of-a-kind ring for your girl, using only what she would prefer.