If it’s wedding ring, the marketing has been about how the purity of the diamond is symbolic of the purity of one’s love. So picking an uglier, but purer, diamond then coild be about prioritizing love over beauty or whatever in that person’s head.
So not necessarily exclusive to people wanting to present themselves as wealthy, that sort of emotional manipulation convinces broke people to blow their savings on a ring all the time.
And the absurd thing is that we probably should use natural diamonds for jewelry, as it is a waste to synthesize such a common rock. If they weren’t hoarded and aggressively marketed as something you have to have, they could just be a neat rock some people are into, there wouldn’t be child slaves dying to dig them out of the ground.