https://en.wikipedia.org/wiki/Veblen_good

A Veblen good is a type of luxury good, named after American economist Thorstein Veblen, for which the demand increases as the price increases, in apparent contradiction of the law of demand, resulting in an upward-sloping demand curve. The higher prices of Veblen goods may make them desirable as a status symbol in the practices of conspicuous consumption and conspicuous leisure. A product may be a Veblen good because it is a positional good, something few others can own.

That said, part of the problem with lab-grown diamonds is that they're not competing against a rare commodity. They're competing against a powerful vertically integrated cartel. There isn't any real diamond shortage, just a supply-side monopoly. There isn't a natural high demand for diamonds, just a market saturated with aggressive advertising. There isn't a wholesale diamond exchange judging the rocks objectively on their quality, just a series of elaborate marketing gimmicks and scammy sales goons trying to upsell you.

Diamonds have always been a racket. The one blessing of manufactured diamonds is that they're no longer a racket putting market pressure on industrial grade diamond equipment. But the jewelry exists to separate gullible superficial status-fixated people from their money. Ethics was never part of the equation.