If it were me, I’d be upset that I didn’t rate high enough in his esteem to deserve an antique diamond - now, before anyone lobs a holy hand grenade at me, hear me out....
My intendeds should *know me* well enough to know there’s no Ersatz anything (no fake marble, no hair dye, no pseudo-antiques, no imitation crab....) and frankly my taste should be really obvious to those close enough to claim to love me!
Any suitor who would expect me to consider MMD anything but a placeholder, clearly doesn’t know me. It would be a puzzling and disappointing choice.
Whenever a beloved has given me jewellery, they have always been flustered in the most adorable way, hoping it pleased me, and babbling sweetly that if I didn’t like it I could change it, etc., and if the guy in the OP’s anecdote didn’t stumble on his words, stammering and blushing, and really truly trying to please her, then he’s a cad.
And not the kind of CAD that PSers love to get!