Media playback is unsupported on your device Media caption Scientists studying Australian diamonds hope to discover the secret to their pink colour

They're one of the world's rarest jewels - but nobody knows for certain why pink diamonds are pink.

That hasn't stopped investors from snapping them up at auction and sending prices skyrocketing. In October a new world record was set at a Sotheby's sale in Hong Kong when an 8.41-carat pink diamond sold for $17,768,041 (£11,438,714) - more than $2.1m (£1.8m) a carat.

"Everybody's talking about them, and everybody loves them," says Jeffrey Post, curator of the National Gem and Mineral Collection at the Smithsonian's National Museum of Natural History in Washington, DC. "Yet you can't tell people why they're pink."

Other diamonds get their colour from chemical impurities that absorb light. Yellow diamonds contain traces of nitrogen, and blue diamonds contain boron. But no similar impurities have been found in pink diamonds, leading scientists to speculate that the colour may be the result of some kind of seismic shock that altered the stone's molecular structure.

It's now hoped that a cache of brown and pink diamonds from the Argyle mine in Western Australia may solve the mystery. The mine, owned by Rio Tinto, is the world's largest source of pink diamonds, even though they're so rare that only a few are produced each year.

Image copyright Smithsonian Image caption A collection of Australian diamonds could solve the mystery of why pink diamonds are pink

As well as revealing what makes them pink, scientists hope that studying the diamonds will tell them more about the history of the planet.

Diamonds are the Earth's messengers, says Post. "They come from a hundred miles below the surface and tell us about a part of the Earth that we can't visit. They're also giving us a peek back in time because most diamonds formed about two to three billion years ago.

"Each one is a time capsule, and the pink diamonds, because they're different from all the other diamonds, have a different part of the story to tell."

Scientists have already examined the Argyle diamonds using a mass spectrometer to try to find any trace of impurities that may be causing the pink colour. The machine agitates the diamonds and analyses the chemical structure of the atoms that are released.

"There is no impurity that we've been able to associate so far with the pink colour in diamonds," says Post. "Spectroscopic measurements don't show you any additional features that you can ascribe to a particular colouring agent."

They've also used a focused ion beam to cut a tiny trench in the surface of the diamonds and remove a sliver that can be measured under a powerful electron microscope. They've discovered that most pink diamonds are not uniformly pink but have pink zones that alternate with clear areas.

The zones, known as twin planes, were formed by some kind of shock - possibly the result of volcanic activity that propelled the diamonds to the surface or from something that happened to them as they were being formed deep underground.

Image copyright Getty Images Image caption This 59.6-carat pink diamond was auctioned by Sotheby's in 2013

"The twin plane itself should not give rise to colour," says Post. "But we think when those twin planes form, and slide back and forth, one against the other like a fault plane, that certain kinds of defects formed. The defects give us the pink colour. But what we've not been able to do yet is find the specific kind of defect."

Although pink diamonds are among the most valuable jewels today, 20 years ago they were little more than a geological curiosity. Sales have been driven by savvy marketing and a growing appreciation of their uniqueness.

"It really comes down to the rarity," says Richard Revez, a gem expert at Florida-based Kravit Estate Department. "When you talk about coloured diamonds, they're already in the elite 1% produced in the world. Pink diamonds are the 1% of the 1%."

He says the most sought-after diamonds are actually red, but orange, green, blue and yellow are highly desirable. An orange diamond attracted the highest price paid per carat for any diamond at auction last year, selling for $35m, or $2.4m a carat.

"We've craved diamonds for millennia," says Revez. The first gems were probably discovered on river banks in India, but their existence is recorded in Greek and Roman history. "It was believed there was a vein that ran directly from the heart to the ring finger - that's why we wear (diamonds) on our ring fingers. And Cupid's arrows were tipped with diamonds to pierce the heart easier," he says.

Archduke Maximilian of Austria is believed to have started the tradition of diamond engagement rings among the upper classes when he presented one to Mary of Burgundy in 1477.

But it wasn't until the 1950s that international standards to grade diamonds were set by the Gemological Institute of America (GIA), a classification system that is still used today.

Image caption Physicist Keal Bryne seeks to determine the molecular structure of the gems

But only science can reveal why pink diamonds are pink.

Pink diamonds can be artificially created, says the Smithsonian's Post. And the only way to tell if it's a synthetic stone is to understand what causes the colour to occur naturally.

"Then I can tell you for sure that that is a diamond that came out of the earth as opposed to one that came out of somebody's laboratory. It can make the difference of millions of dollars in the value of a single diamond, knowing whether it is a natural pink or not."