Diamonds lose their sparkle, as women choose colour
Over the past eight years, the price of rubies and sapphires appears to have increased faster than that of diamonds.
Coloured stones are, of course, much more affordable than their pricier, harder, relatives.
And millions more people, especially in Asia, want to buy them.
But could another explanation be that many more women are now buying gems for themselves?
That is the theory being put forward by one of Hatton Garden's most established jewellers.
Vashi Dominguez, the founder of the firm Diamond Manufacturers, believes prices are now being driven by the choices that women make.
When men choose gemstones for their partners, they often go for diamonds, he says. But when they do the choosing themselves, many women prefer at least one coloured gemstone in a piece of jewellery - a ruby, a sapphire, or an emerald.
They have also received encouragement from celebrities like Angelina Jolie and Penelope Cruz, who can afford diamonds aplenty, but who often choose to wear sapphires and emeralds instead.
Not to mention the Duchess of Cambridge, with the engagement ring she inherited from Princess Diana.
"I have a thing about colour - amethysts and rubies," says Georgina Rycyk, a marketing manager in Hatton Garden, the centre of the UK's jewellery trade.
She has just bought herself what she describes as a "sizeable" ruby.
"I just really like colours. My budget is not in diamonds, although it might be one day," she says.
Many customers appear to think along similar lines. Even if they still want one diamond in an engagement ring, they also want a splash of colour.
"What women want is a vintage setting, with coloured side stones," says Ms Rycyk.
Demand for coloured stones has increased threefold since 2010, according to Diamond Manufacturers' Mr Dominguez.
As a result, the company is selling fewer diamonds.
"It's because women are driving the market," he says, "even in China."
If men choose jewellery, he says, they tend to buy a ring with a single diamond in it, otherwise known as a solitaire.
"Buying jewellery is normally a very daunting experience for a man," he says.
"Often they don't even know the ring size, while women really know what they want. Some women even say, 'Can you change the diamonds to sapphires?'"
Georgina Rycyk believes most men are fishing in the dark when it comes to choosing jewellery for a wife or girlfriend.
"If I was to go into a sports shop to buy a pair of football boots, I would have no idea what to look for," she says.
"It's the same when men try to buy jewellery."
The Kate effect
The market price of coloured stones appears to reflect their growing popularity around the globe.
Gemval, which claims to offer the world's first online valuation tool for gemstones, reckons that the price of sapphires rose by 37% between January 2006 and January 2013.
Rubies rose by 47% over the same period.
Emeralds rose significantly in 2005, but between 2006 and 2013 only rose by about 6%.
According to the RapNet Diamond Index (RAPI), the price for one-carat diamonds has risen by 32% since 2006.
In other words, sapphires and rubies have outpaced diamonds over the past seven years.
But the science of pricing such gems is not exact.
"The problem with coloured stones is that there is no grading," says Martin Rapaport, whose company produces the RapNet index at its headquarters in Israel.
"But if you want a top quality ruby or emerald, they will compete with diamonds at that level," he says.
Jaspreet Chawla, a London-based gemmologist, believes the price rises reflect a growing trend in certain markets.
"In the US and Germany there is a more pronounced move towards coloured stones," she says, "increasingly so after Kate Middleton wore a sapphire.
"But the UK is still diamond-dominated."
Celebrities may indeed be helping to make coloured gemstones fashionable.
Penelope Cruz and Elizabeth Hurley both have sapphire rings, while Halle Berry and Angelina Jolie are fans of emeralds.
Chloe Fuller, a marketing manager from south-west London, bought herself a blue sapphire ring for £900, and a sapphire necklace for £700, after inheriting money from her grandmother.
She dreams of being able to afford a pair of sapphire ear-rings to match.
"Before Kate, everyone was buying solitaires," she says.
"But now a lot of my friends have sapphire rings like this. They're classic, and they won't date.
"I also wear lots of blue, so it goes with what I wear," she says.
But she dismisses suggestions that men are not good at choosing jewellery for their partners.
She brandishes a bracelet with a miniature ice cream attached.
"My boyfriend gave me this, because I like ice cream. I like it, because it's what he likes.
"It's the thought that counts."