PEACOCKS strut; bowerbirds build lovenests; spiders gift-wrap flies in silk. Such courtship rituals play an important role in what Charles Darwin called sexual selection: when the female of a species bears most of the costs of reproduction, males use extravagant displays and gifts to demonstrate their “reproductive fitness” and females choose between them. For human males, shards of a crystalline form of carbon often feature. A diamond engagement ring signals a man’s taste, wealth and commitment, all to persuade a woman that he is a good bet.

This particular courtship gift was dreamed up by an ad agency for De Beers, the cartel that sold almost all of the world’s diamonds throughout the 20th century. In the 1930s it started to promote a link between diamonds and marriage. Diamonds’ unmatched hardness would symbolise love’s endurance and their “fire”, or brilliance, its passion. Two months’ salary, the firm suggested, was what the ring should cost—a good investment since, as the admen said, “A diamond is forever.”

Now, that promise is dimming (see article). Though a growing Chinese middle class will probably prop up demand for a while, millennials in Western countries seem keener on memorable experiences than on bling. Diamonds’ image has been blemished by some being mined in warzones and sold to pay for the fighting. Meanwhile, laboratory-grown “synthetic” diamonds, long fit only for industrial use, are becoming good enough to compete with gems from out of the ground.

But the long-term threat to diamonds’ lustre is more surprising: that their price could plummet. In recent years regulators (and market forces) have undermined De Beers’s cartel by limiting the share of other producers’ stones that it can buy. Now responsible for just a third of global sales, the company can no longer manage supply by stockpiling gems when demand turns down. It is spending less on advertising, since it no longer gets the lion’s share of the benefits. But the very value of diamonds lies in being scarce and coveted—that is, costly. In the jargon, they are “Veblen goods”, named after a 19th-century economist: prestige-enhancing trinkets for which a higher price encourages buyers. With most products, lower prices increase demand; with diamonds, they could kill it.

Greater equality for women might seem to render male-courtship displays redundant. But mating preferences evolved over millennia and will not change quickly. If diamonds were to cease being a way to signal a man’s marriageability, what might take their place?

A different gift, perhaps. In China skewed sex ratios mean that a prospective bridegroom must own an apartment and shower his future in-laws with cash. But a glittering stone goes to the woman, not her family. And it is more than a gift: it is a status symbol, demonstrating that even as a man approaches the expenses of married life, he can still splash out on a bauble. Or a man could rely on more generic forms of display, such as a fancy degree, good job or sharp suit. But these can impress one woman as easily as another, or several simultaneously. He must show commitment—a need not unique to courtship. Salvadoran gangsters get extravagant tattoos; Japanese yakuza cut off a fingertip. These visible signs of allegiance make it hard to defect, and impose heavy costs. But as marriage proposals they would fall short. Few women would feel proud to carry around their fiancé’s severed pinkie.

Love is a multifaceted thing

Many millennial women seek a mate who is creative, charitable and earns enough not to live with his parents. The millionaire founder of a startup that makes an app to teach yoga to orphans would be ideal. As a token of his commitment, a suitor might offer the object of his affections 51% of his shares—so much nicer than a joint bank account. Less eligible men could offer instead to link Uber accounts, thus entwining the couple’s reputations: their joint five-star rating would be at risk if either misbehaved. Uber-linking would also allow each to keep track of the other’s whereabouts, discouraging infidelity. Whatever ultimately replaces diamonds, it will surely be digital, not worn on a digit.