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Theyre one of the worlds rarest jewels – but nobody knows for certain why pink diamonds are pink.

That hasnt stopped investors from snapping them up at auction and sending prices skyrocketing. In October a new world record was set at a Sothebys 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.

Everybodys talking about them, and everybody loves them, says Jeffrey Post, curator of the National Gem and Mineral Collection at the Smithsonians National Museum of Natural History in Washington, DC. Yet you cant tell people why theyre 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 stones molecular structure.

Its 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 worlds largest source of pink diamonds, even though theyre so rare that only a few are produced each year.

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 Earths messengers, says Post. They come from a hundred miles below the surface and tell us about a part of the Earth that we cant visit. Theyre 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 theyre 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 weve been able to associate so far with the pink colour in diamonds, says Post. Spectroscopic measurements dont show you any additional features that you can ascribe to a particular colouring agent.

Theyve 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. Theyve 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.

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 weve 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, theyre 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.

Weve 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 – thats why we wear (diamonds) on our ring fingers. And Cupids 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 wasnt 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.

But only science can reveal why pink diamonds are pink.

Pink diamonds can be artificially created, says the Smithsonians Post. And the only way to tell if its 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 somebodys 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.

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