Instead of asking random strangers, this is something you need to speak to your partner about. Most women have a good idea as to what they want, and this should be a mutual decision, even if it is a gift from you to her. Other gifts are not presumed to be worn for a lifetime- it is not smart to go into this blindly.
It needs to be made of materials that will actually last for that lifetime, so you need to find something made of gold, white gold (actually an alloy) or platinum, and if it has stones, they should be hard enough not to break or wear down. It could be a quite small diamond or gemstone ring for a few hundred dollars, or a larger diamond, ruby or sapphire for thousands.
The key is that it be something she is proud to wear, and in a style she likes, but not so expensive that it is using money you actually need for your future or present. While it once was seen as a payment for her virtue, and a demonstration that you could afford to keep her, now it is more symbolic.
Ask her to marry you first, and then take her ring shopping- after you agree on how much you are able to spend, and what she actually wants. You may find she doesn't want a big expensive diamond at all, and as a couple, it's time you learned how to make such a decision together.