The spell is called herd mentality. People don't like to be losers, so women wear shiny stones and men drive cool cars. Both want to send the message "I'm not a loser". Appeal to science has no bearing on the herd opinion. Your gf was basically terrified that her friends would laugh at her lab grown diamond and her weak scientific arguments won't raise her ingroup status. Edit: I'd add a snarky observation that a diamond is essentially a notarized letter of "love" where the shop gets paid as the notary and your gf gets the proof of your deposit. The stone itself isn't worth much.