Sure, and we all know it. But from her perspective, you probably waste all kinds of time and money on other inane bullshit for yourself. So if you actually cared, you wouldn't mind spending a bit so she gets a shiny rock to wear around. Why shouldn't she get this one thing?

Also, how is this any different from fashion? Both are nearly useless raw materials that have been transformed and shaped into something people will pay money to own and wear (signaling). For diamonds, you get conflict in Africa. For fashion, you get sweatshops in Asia.