That invention is surprisingly recent: Epstein traces its origins to the discovery of massive diamond mines in South Africa in the late 19th century, which for the first time flooded world markets with diamonds. The British businessmen operating the South African mines recognized that only by maintaining the fiction that diamonds were scarce and inherently valuable could they protect their investments and buoy diamond prices. They did so by launching a South Africa–based cartel, De Beers Consolidated Mines, Ltd. (now De Beers), in 1888, and meticulously extending the company's control over all facets of the diamond trade in the ensuing decades.

Most remarkably, De Beers manipulated not just supply but demand. In 1938, amid the ravages of the Depression and the rumblings of war, Harry Oppenheimer, the De Beers founder's son, recruited the New York–based ad agency N.W. Ayer to burnish the image of diamonds in the United States, where the practice of giving diamond engagement rings had been unevenly gaining traction for years, but where the diamonds sold were increasingly small and low-quality.

Meanwhile, the price of diamonds was falling around the world. The folks at Ayer set out to persuade young men that diamonds (and only diamonds) were synonymous with romance, and that the measure of a man's love (and even his personal and professional success) was directly proportional to the size and quality of the diamond he purchased. Young women, in turn, had to be convinced that courtship concluded, invariably, in a diamond.

Ayer insinuated these messages into the nooks and crannies of popular culture. It marketed an idea, not a diamond or brand:

Movie idols, the paragons of romance for the mass audience, would be given diamonds to use as their symbols of indestructible love. In addition, the agency suggested offering stories and society photographs to selected magazines and newspapers which would reinforce the link between diamonds and romance. Stories would stress the size of diamonds that celebrities presented to their loved ones, and photographs would conspicuously show the glittering stone on the hand of a well-known woman. Fashion designers would talk on radio programs about the "trend towards diamonds" that Ayer planned to start. ... In its 1947 strategy plan, the advertising agency ... outlined a subtle program that included arranging for lecturers to visit high schools across the country. "All of these lectures revolve around the diamond engagement ring, and are reaching thousands of girls in their assemblies, classes and informal meetings in our leading educational institutions," the agency explained in a memorandum to De Beers. The agency had organized, in 1946, a weekly service called "Hollywood Personalities," which provided 125 leading newspapers with descriptions of the diamonds worn by movie stars. ... In 1947, the agency commissioned a series of portraits of "engaged socialites." The idea was to create prestigious "role models" for the poorer middle-class wage-earners. The advertising agency explained, in its 1948 strategy paper, "We spread the word of diamonds worn by stars of screen and stage, by wives and daughters of political leaders, by any woman who can make the grocer's wife and the mechanic's sweetheart say 'I wish I had what she has.'"

In the late 1940s, just before my grandfather started hunting for his diamond ring, an Ayer copywriter conceived of the slogan that De Beers has used ever since: "A Diamond Is Forever." "Even though diamonds can in fact be shattered, chipped, discolored, or incinerated to ash, the concept of eternity perfectly captured the magical qualities that the advertising agency wanted to attribute to diamonds," Epstein writes. A diamond that's forever promises endless romance and companionship. But a forever diamond is also one that's not resold. Resold diamonds (and it's maddeningly hard to resell them, as Epstein's article details) cause fluctuations in diamond prices, which undermine public confidence in the intrinsic value of diamonds. Diamonds that are stowed away in safe-deposit boxes, or bequeathed to grandchildren, don't.