Should I get my girlfriend a promise ring?

I really am in love with my girlfriend, we've been together for 6 months now and I think she's the one. I've been through ups and downs in past relationships and I've never felt this way about anyone, I'm 18 she's 17, she says she wants to be with me forever too. So I want to know would it be crazy for me to buy a promise ring for her? I'd appreciate honesty thank you!(: