*mild facepalm*
Value and worth are collectively determined things. If enough people believe something has great worth then it does. Period. Everything else is irrelevant. It will even have worth to someone who regards it as worthless because it gives them leverage over the people who believe it has worth.
I bet if someone gifted you a whole suitcase full of completely legal, high grade diamonds you wouldn't say, "Oh, diamonds, these are so worthless, lol, I'm gonna chuck 'em in the bin." You'd say, "Fuck yeah, diamonds! These things are worth a packet!".
Also, status symbols are useful. You can ***** about how shallow it is until the cows come home, but if something raises you in the eyes of your peers then it is useful.
As for whether or not I'd use diamonds to propose... eh, never really given it any thought.
Actually, it just occurred to me, I'd probably prefer to use an opal. I just think they're prettier: