J. Courtney Sullivan is the author of “The Engagements,” a novel.

This past week, my aunt forwarded me an e-mail from one of the biggest jewelers in Boston. The subject line: “Chocolate Melts, Flowers Wilt, Diamonds are Forever! Give a lasting gift of love this Valentine’s Day.”

In the body of the e-mail, my aunt asked: “What would Frances say?”

Frances Gerety, whose life and work I chronicled in my novel “The Engagements,” spent her career at the Philadelphia advertising agency N.W. Ayer, where she dedicated herself to one client: De Beers. From 1943 through 1970, she wrote all of the company’s ads, including the iconic tagline “A Diamond Is Forever.” If you’re planning to give — or hoping to receive — a diamond this Valentine’s Day, you have Frances Gerety to thank. Or blame.

When Gerety first suggested the line at a routine morning meeting in 1947, her colleagues in the copy department (all of them men) argued that it didn’t really mean anything. The word “forever” wasn’t even grammatically correct.

Gerety didn’t think the line was one of her best, either. “I shudder to think of what might have happened if a great line had been demanded,” she wrote in a letter 40 years later. “Every copywriter in the Department coming up with hundreds of lines and the really great one lost in the shuffle.”

“A Diamond Is Forever” has appeared in every De Beers engagement advertisement since 1948. And in 1999, Advertising Age proclaimed it the slogan of the century . “Before the DeBeers mining syndicate informed us ‘A Diamond Is Forever,’ associating itself with eternal romance, the diamond solitaire as the standard token of betrothal did not exist,” the magazine explained. “Now, thanks to the simple audacity of the advertising proposition, the diamond engagement ring is de rigueur virtually worldwide, and the diamond by far the precious gemstone of choice.”

De Beers hired N.W. Ayer in 1938 to make Americans fall in love with diamond engagement rings. Diamonds were seen as an extravagance for the wealthy, and sales, already declining for more than two decades, had plummeted during the Great Depression. Ayer was in the unique position of having to sell to the masses a product that they most likely did not want and definitely did not need. Internal Ayer documents later observed that the campaign had required “the conception of a new form of advertising, which has been widely imitated ever since. There was no direct sale to be made. There was no brand name to be impressed on the public mind. There was simply an idea — the eternal emotional value surrounding the diamond.”

The brilliance of “A Diamond Is Forever” lies in its emphasis on both eternity and sentiment. It was the distillation of a theme Gerety had been writing for several years. Her previous ads — in publications including Look, Vogue, Life, Collier’s, Harper’s Bazaar and the Saturday Evening Post — often focused on permanence and timelessness. A series of honeymoon images said, “May your happiness last as long as your diamond.” Another counseled, “Wear your diamonds as the night wears its stars, ever and always . . . for their beauty is as timeless.” There were ads about couples parted by war, hinting that a diamond might be the key to immortality. And ads that appealed to male ego, likening the purchase of a diamond to the founding of a city.

Timelessness is as much about the past as the future. In annual reports to De Beers starting in the ’40s, Ayer often made mention of advertising focused on “the engagement diamond tradition.” There actually was no tradition, but the more they told the story, the more the public came to see it as fact.

The tagline represents an ultimate (and ultimately impossible) promise: If you buy this object, your love will never end. Without a diamond, Gerety’s message seemed to warn, there’s no such thing as forever. The warning worked. In 1951, Ayer informed De Beers that “jewelers now tell us ‘a girl is not engaged unless she has a diamond engagement ring.’ ” The same year, eight out of 10 American brides got a diamond. That number has stayed relatively consistent ever since.

Through the years, Gerety’s slogan has remained a constant as the copy and art have evolved to keep up with the times. Like old family photos, some ads speak to a specific moment:

1971: “My church may be a meadow. And my wedding won’t be my mother’s. But when it comes to my man and the diamond his love gives me, I want all things that women have always dreamed of. A Diamond Is Forever.”

1980: “I know she loves rock-n-roll. So I rolled out a magnificent rock. A Diamond Is Forever.”

The line was sometimes tweaked to convey a particular message. In the ’60s, when De Beers wanted to persuade customers to buy a surplus of unfashionably tiny stones, Gerety’s campaign shifted the focus from “the bigger, the better” to “size isn’t everything.” Some ads during this period seemed a bit desperate. “The diamond is forever,” they said. “Even the small diamond.”

Eventually, the slogan spread to television, in the much-mocked “Shadows” campaign of the 1990s. By then, Gerety had long since retired and De Beers had taken its business to the J. Walter Thompson agency. But the line lived on.

Today, “A Diamond Is Forever” is studied in business and marketing classes at New York University, the University of Massachusetts at Amherst and the University of Pennsylvania, among others, as an example of innovative techniques, complexity of message and sheer longevity. (Another of Ayer’s major clients, AT&T, had 13 different slogans over the course of the 20th century.) One professor told me that many of her students were familiar with the phrase but never knew it was an advertising slogan.

The best lines seamlessly blend into the culture, each reinforcing the other’s importance. “A Diamond Is Forever” gained elevated status in popular culture when it inspired the title of Ian Fleming’s 1956 novel, “Diamonds Are Forever,” the fourth in the James Bond series. In 1971, the film and the Shirley Bassey song of the same name brought even more attention to diamonds. The elaborate jewels featured in the movie were on loan from the London jeweler David Morris, by then a two-time winner of De Beers’s annual Diamonds International Award for innovative jewelry design.

In 2005, Kanye West sampled Bassey’s chorus in his “Diamonds From Sierra Leone.” The song makes clear that it is anti-blood-diamond. But it is not anti­diamond. A line of text at the end of the music video might even be taken as an advertisement. It says, “Please purchase conflict free diamonds.” West’s affinity for diamonds was on full display last fall, when he proposed to Kim Kardashian with a massive 15-carat diamond ring.

De Beers now owns only 35 to 40 percent of the world’s diamond supply. When the company had a controlling share of the market, anti-trust laws prevented it from making direct sales in the United States. But giving up its monopoly status meant that it could have retail stores and product lines, including the Forevermark brand. Forevermark stones are trendy and expensive, and they come with the promise that they hail from “a small number of carefully selected mines that are committed to the highest business, social and environmental standards.” The name Forevermark is, of course, a reference to Gerety’s tagline, which has seen its most modern incarnations in the De Beers Web site ADiamondIsForever.com and Forevermark’s use of the Twitter hashtag #forever.

On the occasion of the 50th anniversary of N.W. Ayer and De Beers working together, Gerety was honored at a celebration in London. When asked by the agency’s public relations team to reflect on all she had written for the campaign, she said she wouldn’t change a word.

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