Diamonds, in the words of one of the world’s most famous marketing slogans, are supposed to be forever.
But a growing fashion for engagement rings with coloured stones, such as rubies and emeralds, is threatening to undermine their pre-eminence.
When Frances Gerety, a young advertising copywriter, coined the ‘forever’ slogan in 1947, she helped established the status of diamonds as a must-have accessory.
That, coupled with the De Beers diamond company’s marketing campaign which invented the ‘rule’ that a man ought to spend at least two month’s pay on his fiancee's engagement ring, reinforced the cachet of diamonds as a gift of rare and eternal beauty.
But experts say that the rise of the Millennials, with their more cost-conscious and individualistic shopping habits, along with their growing demands for more ethical sourcing of products, have led to diamonds losing their sparkle.
New research by insurance house, Allianz, shows that the majority of men now fork out far less than two to three month’s pay on a stone, with an average spend of £573, with middle-class couples preferring to save their money for future school fees, spiralling house prices and adventurous holidays.
With cheaper emeralds and rubies growing in popularity just one in five women can expect to receive a ring, costing between £750 and £3,000, Allianz found.
Anusha Couttigane, a senior analyst at Kantar Retail, said: "A generation of marital age people are now prioritizing other things such as weddings, housing and the cost of having children, rather than splashing out on a really expensive ring.
“There is still a lot of demand for solitaire diamond rings, but there has been growth in non traditional designs which use a range of cheaper, coloured stones too.”
At the same time tastes in jewellery have changed and coloured stones are seen as providing a more individualistic and distinctive alternative to the once ubiquitous white diamond.
Joanna Hardy, a jewellery expert for the BBC's Antiques Roadshow, said: "The great thing about coloured stones is that you can buy something really fabulous looking and most people will have no idea how much it cost. This is a big selling point for people who perhaps don't have as much to spend on a ring and don't want others to know exactly what they've spent on it."
Young people, says Ms Hardy, are steering clear of generic style rings and instead have a tendency to choose unique or unusual rings, which they see as reflecting their individual personality.
Ethical shopping has also dented the market for the traditional rock, with Leonardo Di Caprio’s film Blood Diamond alerting consumers to the human cost of extracting them from the earth and the conflicts the tread fuels.
Jewellers, including the high end designer Chopard, are now promoting "traceable" emeralds through marketing campaigns aimed at eco-conscious consumers.
In 2013 Chopard launched its "Green Carpet Collection", the first high-jewellery range made with Fairmined gold and ethically sourced diamonds. More recently it has partnered with Gemfields, whose Zambian emeralds were worn by Julianne Moore at the Cannes Film Festival in May.
Meanwhile the growing number of second marriages has seen a boom in secondhand and antique rings, which tend to be around half the price of new pieces.
The number of marriages has been rising since 2013, partly thanks to a growth in couples in their late 50s and 60s getting hitched for a second or third time.
Department store John Lewis said the trend has led demand for its range of antique jewellery to boom.
A spokeswoman said: "Second hand engagement rings are increasing in popularity, particularly with people getting married later in life, or for the second time. People who aren't marrying for the first time are often looking for a simple design having chosen a large stone, or statement ring the first time."
Sloane Square-based jewellery designer Kiki McDonough, added: “There is a huge trend to choose coloured stones for an engagement ring, particularly for second time marriages as people are looking for something totally different.”
Perhaps it’s just as well trends are shifting away from white diamonds.
The supply of new diamonds - formed by huge geological forces beneath the surface of the earth more than a billion years ago - is expected to peak in the next few years, before it starts to decline.
In September De Beers opened its Gahcho Kué mine in Canada’s Northwest Territories, where at full production it will extract more than 12,000 carats (2.4kg) of diamonds each day.
But by 2030 the mine’s supply diamonds will have been exhausted and it will shut. De Beers has no plans to open other new mines.
Diamonds, it seems, really aren’t forever.