How do coloured diamonds form?

On display in a lofty Museum gallery is a small, glittering triangle of diamonds.

It is the Aurora Pyramid of Hope, a collection of 296 naturally coloured fancy diamonds.

They are extremely rare. For every 10,000 colourless and near-colourless diamonds, only one is classed as fancy, meaning outside the normal colour range.

New York diamond collectors Alan Bronstein and Harry Rodman spent years searching for and assembling the gemstones. Now the collection is one of the largest of its kind.

The diamonds we know best are sparkling and colourless, but this collection ranges in colour from yellow-green to pink.

Mike Rumsey, Senior Curator of the Mineral collection at the Museum, explains how the stones naturally developed such bold shades:

'Perfectly formed diamonds composed only of the element carbon, with no chemical impurities, no internal defects and no extra particles would be completely colourless.

'But nature rarely produces perfect objects. In the case of diamonds, it is these imperfections that cause colour.'

How are diamonds formed?

These precious gems are formed deep in the Earth's mantle - the layer between the crust and the planet's core.

The mantle experiences extremely high temperatures and pressures, up to 160 kilometres below our feet.

No scientist can travel that far underground, so we don't know exactly how diamonds are created. But we know they are formed of carbon atoms coming together under intense pressure and heat to form crystals.

The stones that we mine today are not this deep. They were brought up from inside the planet by huge and ancient volcanic eruptions.

Mike says, 'The different geological environments and formation processes are what causes imperfections in the stone - and these are what give diamonds their colour.'

Many diamonds actually show very pale colours, most often in shades of yellow, brown and grey. They are rejected as gemstones and used in industry instead, often as abrasives.

However, on the rare occasions when the colour is more intense, they become extremely desirable. 

Yellow and brown diamonds

The majority of natural diamonds contain nitrogen atoms as impurities in their internal structure. These impurities produce a yellow or brown colour.

Mike says, 'Because nitrogen is so widespread, it is no surprise that pale tints of yellow and brown are the most common coloured diamond.'

The colour is affected by how the nitrogen is scattered through the stone.

If single nitrogen atoms replace carbon, the diamond can be bright canary yellow. If the nitrogen atoms are clustered around missing parts of the internal structure (tiny holes that mineralogists call defects) a paler yellow called cape is produced. 

These diamonds also glow fluorescent blue under ultraviolent (UV) light. The Aurora Pyramid of Hope collection contains several diamonds that do this.

Brown diamonds are caused by the crystal becoming deformed. Their internal structure is warped, creating tiny holes which cluster and affect the way light passes through the stone.

Deep blue and green diamonds

The chemical element boron makes diamonds go blue when it finds its way inside them.

Blue diamonds only need a tiny amount of boron to produce colour, but they are much rarer than yellow ones.

A green diamond means it has been near radioactive mineral at some point in its history.

Mike says, 'Depending on the type of radiation that the diamond was exposed to, the green colour may just be on the surface, or evenly spread throughout the entire crystal.

'It is easy to irradiate objects, so green diamonds are produced synthetically too. Sorting a natural example from one produced in a lab is not always possible.'

The rarest hues

Naturally coloured red diamonds are extraordinarily rare, and scientists are still not completely sure what causes their rich colour.

It is thought that the colour in red and pink diamonds is caused by a change in the structure of the crystal called plastic deformation, possibly in combination with another internal defect.

This distortion changes the way light interacts with the atomic structure of the diamond as it passes through the stone, causing it to look red or, more often, pink.

Purple diamonds exist but are almost never found because one stone must be exposed to both the red and blue causes of colour change to create the blend.

Mike says, 'Perhaps the most intriguing of all the coloured diamonds is the chameleon. At the apex of the Pyramid of Hope is a six-carat diamond that can change colour. It will go from olive green to yellow when it is exposed to light after a period of time in the dark, or when it is gently heated.'

The internal mechanisms, impurities or imperfections responsible for the diamond's change in colour are still being researched.

Mike says, 'It is probably due to a combination of factors and includes a significant presence of both hydrogen and nickel atoms.'

See the display

The Aurora Pyramid of Hope is on permanent display in the Vault.