Earlier quoted context omitted.
She even told you her true requirement, which is to look good for her girlfriends. (Watch what happens when ladies announce an engagement or look for the ring shot on Facebook, then tell me I'm off base.) You did not sell her a product which would make her the envy of her girlfriends. "Cheaper than their rings" is the opposite of a victory condition. Don't sell cheap. Sell authentic, sell natural, sell one-of-a-kind,…
> Don't sell cheap. Sell authentic, sell natural, sell one-of-a-kind, sell meaning, sell status, sell a story, etc. Basically: sell it better than De Beers has been doing for over half a century. i.e. best of luck!
Your advantages: you actually know the girl, you apparently know what makes her tick well enough to make her fall in love with you, you know everyone in her Circle of Girlfriends reference set (or would if you cared to), you're not a sclerotic megacorp, you've got copious permission marketing assets to her and her CoGs, and you've actually got a soul, which should count for something.
Besides, this is HN. Scrappy startup versus megacorp which hasn't innovated since the first world war? Oooooh, bring it, this should be fun.