Ben Caruana, 29, of Cotham Road, Kew, spent three weeks visiting jewellers along High Street before proposing to his partner Mia, trying to reconcile his budget with what he'd always understood to be the rule: a ring should cost roughly two months' salary. At the third store, a jeweller mentioned, almost as an aside, that the rule wasn't really a rule at all. It was a line from a 1947 advertising campaign.

Ben went home and looked it up. The campaign, "A Diamond Is Forever," was written for the mining company De Beers by the agency N.W. Ayer, which had been hired specifically to manufacture demand for diamonds at a time when sales had slumped and rings were increasingly set with other stones. The agency didn't just coin a slogan. It invented a social expectation from scratch, placing diamond rings in Hollywood films, getting jewellers to recommend a fixed percentage of salary, and steadily increasing that suggested percentage over the following decades until two months became the accepted norm, with no origin older than an ad brief.

For Ben, this was clarifying. He decided, with some pride, that a $1,200 vintage sapphire from a dealer in Malvern would do the job just as well as a $7,000 diamond, and that Mia, who he correctly guessed would find the De Beers story as irritating as he did, would appreciate the ring more for what it symbolised than what it cost.

Mia said yes. She also, three weeks later, posted a photo of the ring to a family group chat, where her cousin, newly engaged herself, replied with a photo of her own 1.8 carat solitaire and the caption "ring twins!" followed by a laughing emoji that Mia did not find especially funny. By the following weekend, under what Ben describes as "logistical pressure rather than a change of heart," the couple had exchanged the sapphire for a diamond upgrade, financed over twelve months, that Ben estimates at closer to three months' salary than two.

Ben says he still believes everything he read about the 1947 campaign. He has simply concluded that knowing something is manufactured and being unaffected by it are different skills, and that he does not currently have the second one.