In 1947, copywriter Frances Gerety invented the tagline "A diamond is forever" on behalf of South Africa's De Beers Group. Dubbed the "slogan of the century", the strapline has been in use ever since, tapping consumers' desire to own a timeless, precious symbol of love.
"It has survived because 'a diamond is forever' summarises that sense of commitment and bond," says Stephen Lussier, De Beers' executive in charge of marketing.
The marketing prowess of De Beers, which at one point controlled 90pc of the diamond market, cemented the ritual of buying a diamond engagement ring in people's minds. But since 2000, the industry has undergone profound change.
In a bid to operate freely in the US, De Beers bowed to antitrust pressure and surrendered its monopoly on diamond sales. It now oversees around 34pc of rough diamond sales worldwide and a new set of players such as Petra, Gem and Lucara have emerged.
At the same time, the Kimberley Process is working to stamp out sales of "blood diamonds" from conflict zones, while producers and retailers are grappling with changing attitudes. A diamond may be forever, but consumers are not.
Can the industry win over a generation of "millennials" with very different demands and expectations?
Despite wobbles in the last few years, the diamond business has retained its lustre.
Global revenue slipped 2pc to $79bn ($59bn) in 2015; slumping demand from China, previously one of the world's fastest growing markets, resulted in an oversupply of demands and a crunch in prices.
De Beers, now 85pc owned by Anglo American, took the unusual step of holding back supply to support prices. It sold 39pc fewer diamonds last year, but could not escape a 58pc drop in earnings.
Nonetheless De Beers remains the largest producer of diamonds by value, followed by Russian giant Alrosa; Rio Tinto, Dominion and Petra follow some way behind the big two.
There were silver linings to 2015's performance.
The US market, which accounts for roughly half of all diamond sales, grew at a healthy 5pc. It remains far ahead of China (17 per cent) and India (7 per cent) in terms of market share, but diamond producers are careful not to take this for granted. The rise of the millennial, defined as someone reaching adulthood around the year 2000, poses a significant challenge.
Millennials haven't been persuaded that a diamond is forever, so they maybe buy another item.
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"In the UK and US, we are noticing that millennial consumers are less enthused about diamonds," says Anusha Couttigane, analyst at Kantar Retail. If they buy jewellery, they are more likely to go for cheaper rocks, or coloured stones, she adds.
"Millennials haven't been persuaded that a diamond is forever, so they maybe buy another item," concurs Des Kilalea, analyst at RBC Capital Markets.
Young consumers are often depicted as disengaged, "digital natives", who view the world through the ever-present filter of an iPhone screen. Perhaps more pertinently, they do not have a great deal of money, and their earning potential is poorer than previous generations -- which is bad news for diamond producers.
Kate, a 30-year-old executive assistant, admits that, engagement rings aside, diamond jewellery does not hold great appeal.
"I'd feel pretty silly wearing something really elaborate to be honest. I would never buy any type of jewellery like this myself -- I'd rather spend my money on something else."
Graphic designer Mike, 31, adds: "I feel they have their place in wedding rings, but I wouldn't save up to buy them as a gift. And a small part of me would question where these diamonds came from, and their history."
The industry is aware that such apathy could be deadly. Lussier points out that young people in the US are in fact active consumers of diamonds, but the industry would be "complacent" if it did not act to shore up its appeal to this group, and encourage repeat purchases beyond engagement rings.
For decades, De Beers bankrolled the marketing of diamonds by up to $150m a year. The collapse of its monopoly meant that diamond marketing fell into abeyance. Now the top seven producers in the world have formed the Diamond Producers' Association (DPA). This summer it has unveiled a marketing campaign under the slogan "real is rare".
"'A diamond is forever' continues to influence consumers but it's a De Beers proprietary asset. We want something crafted around the younger generation," explains Jean-Marc Lieberherr, chief executive of the DPA. Research revealed that younger consumers value experiences over possessions, and enjoy a vast network of "shallow connections" through social media. They are less concerned about upholding rituals and they don't like being told what to do.
The breakthrough for the DPA, Lieberherr explains, was the notion that young people are looking for a deeper connection -- something "real" in an increasingly "digitised world".
"Diamonds can be an ideal metaphor for what's hard to find, hence rare," he says.
"You can't get more real than a diamond; it's been there for billions of years." The DPA's hunch is that the very geology of diamonds can resonate with consumers.
Diamonds can be an ideal metaphor for what's hard to find, hence rare.
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They are, indeed, rare -- though they are by no means the rarest gemstone (painite, with only two examples ever found, holds that honour). Diamonds are carbon crystals formed in the Earth's crust under intense heat millions of years ago.
They are accessible only because they were forced upwards in great eruptions of magma. Today, these diamond-bearing "kimberlite pipes" are found -- and mined -- principally in Russia, Botswana (now home to De Beers), the Democratic Republic of Congo, Australia, Angola, South Africa and Canada.
Lieberherr, who was until January managing director of Rio Tinto's diamond division, hopes that Real Is Rare will inspire a "movement". Despite the focus on young people, its appeal will go wider, he says.
Johan Dippenaar, chief executive of Petra Diamonds and a member of the DPA board, agrees: "I don't see the campaign as just for young people. It embraces everything diamonds offer to consumers across cultural lines."
For Dippenaar, a miner of 25 years' standing, it is an adjustment to begin thinking about how diamonds should be marketed.
"When I started, De Beers sold together with Alrosa. In those years you didn't think about it very much." Miners "need to take some responsibility for the future of our product", he adds, pointing out that diamonds are unusual because they sit somewhere between a commodity, such as coal, and a luxury product, such as a Rolex watch.
The Real Is Rare campaign will be rolled out across radio, social media and the web in the US this autumn.
The DPA's annual budget has doubled to $6m, but it faces a challenge.
"It might work, if enough money is put behind it. And it needs a serious amount of money," says Kilalea.
Young people don't just need to be convinced that diamonds are worth the money, they need to be reassured about their provenance.
"It's fair to say the diamond sector has been under a lot of scrutiny," says Lieberherr. "It's totally warranted. We put out a product that has the promise of purity, and we need to live up to that promise." Leonardo DiCaprio, of all people, has done most to bring the ugly side of diamonds to light with his 2006 film Blood Diamonds, which depicted smuggling during the Sierra Leone civil war of the 1990s.
Younger consumers particularly want to know where that product came from and want a lot more assurance that it has 'done good' on its way.
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At the time of its release, the industry had already taken steps to address the problem, in collaboration with a group of governments, by setting up the Kimberley Process. This system, launched in 2000, is designed to stop rebel groups using diamonds to fund wars against legitimate governments -- but it has attracted criticism.
Global Witness, one of the NGOs that helped draft the Kimberley Process, resigned in 2011 over concerns it did not tackle wider human rights abuses.
"We'd like to see the diamond industry acknowledge that it has a responsibility to respect human rights, directly and indirectly, throughout the supply chain," says the NGO's Alice Harle, who has called on companies to sign up to the OECD's guidelines on human rights due diligence. "If a consumer walks into a shop now and asks the jeweller about how they can be assured of whether or not this diamond is associated with human rights abuses, I think a lot of companies might struggle to answer." Lussier agrees that the diamond industry has to raise its game.
"Younger consumers particularly want to know where that product came from and want a lot more assurance that it has 'done good' on its way," he says, arguing the same applies to any luxury item. In 2008 De Beers launched the high-end Forevermark brand, which applies a tiny watermark to its diamonds to assure buyers that their progress from mine to shop has been completely ethical.
The sector claims it has stepped out of the shadows and embraced transparency. "The industry is changing. Mines are clean and working conditions are good," says Dippenaar.
"Many of us have realised since we started sharing statistics that we've done ourselves a disfavour [by being secretive]."
Lussier adds: "Twenty years ago, we probably marketed secrecy as a sort of mysterious benefit. Now it's all about transparency driven by the importance of protecting the reputation of your product." Diamond producers need to shout louder about the benefits they bring to the countries in which they operate, Lieberherr argues. Botswana, for example, derives 30pc of its GDP from diamonds. "We have to be a lot less shy about diamonds being a force for development and making the world a better place," he says.