My SO is a geologist and likes the idea that Earth created the mineral over millions/billions of years through extreme processes. I'm a technologist, so I kinda like the idea that humans created something "better" using our tools and techniques.
have you bought the diamond yet? it sounds like you guys are one of the rare couples for whom diamonds are particularly romantic. maybe you could buy half of a 'real' and half of a synthetic diamond and combine them?
