Holes give diamonds their colour

Diamonds are said to be a girl's best friend, but how do diamonds come by their shiny and alluring structure? Now a new super-powerful electron microscope that can see individual atoms has enabled scientists to see inside diamonds with unprecedented detail to unlock their colourful secrets.

Diamonds are composed of carbon atoms arranged in a very ordered lattice structure. Each carbon atom is bound to another four carbons in a structure resembling a repeating series of inter-connected pyramids.

But if some of the carbon atoms are substituted by atoms of other elements, the colour of the crystal can be altered. Nitrogen makes the diamond appear yellow while boron will produce a beautiful blue hue. Less appealing is a brownish discolouration seen in some crystals, although diamond experts had originally been baffled by what might be making this happen.

Now, thanks to the installation by the UK's Engineering and Physical Science Research Council (EPSRC) of a new microscope called the SuperSTEM, which is sufficiently powerful to see the positions of individual atoms inside crystals and even to test the strengths of the bonds between atoms, scientists can see for the first time the 3D arrangement of the atoms in diamonds.

This has revealed that brown diamonds contain tiny holes, or voids, only a handful of atoms across, which affect the path of light through the crystal, accounting for the discolouration. But these can be removed by heating the diamond to over 2000°C, which causes the bonds in the carbon lattice to reconfigure themselves, filling in the gaps. The result is a much more impressive-looking stone that can command a higher price in the shops.

But being able to make the less valuable brown diamonds look clear means diamond experts need to know when this sort of treatment has happened to avoid misleading purchasers. "An equivalent stone, one that is brown and one that is colourless, [has] up to a factor of 10 difference in price" explains De Beers UK specialist David Fisher. "A treated stone will only command up to 40-50% of the price of a natural stone that has been untreated."

The new microscope isn't just for looking at pricey gems. Installed at the UK's Daresbury research facility it's one of only a handful of such devices internationally and will be used by scientists to unlock the structural secrets of materials ranging from tissues inside the human body to new polymers destined for outer space.