Married members of HN, what did you do if you think diamonds are utterly ridiclious however due to societal constraints some kind of ring is required for your SO?
Your SO probably doesn't want a logical gift, she wants something sentimental that makes her feel that you care about her. For some people, that's an expensive rock with no utility, for others it might be something different. But applying logic to what's ultimately an emotional, illogical affair isn't always productive.