Diamonds don't last forever ... just a very long time By Damon Shorter Romantics will be devastated to hear that diamonds don't last forever - most don't even make it to the jeweller. Dr Wayne Taylor, diamond expert and mineralogist from the Research School of Earth Sciences (RSES), says most diamonds never see the light of day but are dissolved and recycled back into shifting masses of rock deep within the earth. But some diamonds do last a very, very long time. The oldest are about 3.5 billion years old, or three quarters the age of the earth itself. And for a mineral detective like Dr Taylor, the composition of these archaic rocks provides a window to the earth's geological past. Dr Taylor and PhD student Chuck Magee are examining the history recorded in ancient diamonds using high-powered lasers to slice them into sections and electron beams and other forms of radiation to coax out their chemical secrets. The techniques reveal a lot about the earth's past. Impurities trapped in diamonds from the Argyle site in the Kimberley Ranges of north-west Australia, for instance, suggest the gems formed around 2 billion years ago - when a continental collision forced a neighbouring crustal plate underneath the Kimberley basin. Extreme geological forces created the diamonds 200 kilometres underground at temperatures of 1250°C. Rising plumes of potassium-rich magma then tore them from the mantle rocks and carried them to the surface where they lay for a billion years until human discovery. Mr Magee is most interested in an extremely rare diamond type called carbonardo, which is the hardest known natural material due to its polycrystalline structure. The largest diamond every found was a carbonardo weighing over half a kilogram. However the polycrystalline structure that makes carbonardos so hard also makes them unsuitable as gems. Most intriguing about carbonardos is the mystery of how they formed. "No one knows where these things formed or when they formed," Magee explained. "They just show up in rivers in central Africa and eastern Brazil." Many diamond experts now believe the carbonardos were all formed together before being split in two by continental drift. But as Mr Magee says, "where that source would be ... pick a continent, you've got a 50/50 chance." One theory on the formation of carbonardos, popular with geologists until recently, was that a massive comet impact 2 billion years ago created all the world's carbonardos at once. It was thought the incredible force of the impact, which would have left a crater 500km across, could easily have forged diamonds out of carbon present in the rocks. But earlier this year, when Mr Magee sliced open several carbonardos to study their internal structure, he found the comet theory did not hold up. "From the evidence we have, you can be pretty sure these rocks weren't created by a comet impact," he said. After probing the diamonds using infra-red spectrometry and an electron beam, Mr Magee was able to show some carbonardos had grown in multiple stages, not in a split-second impact. Yet he points out it is easier to discredit someone else's theory than come up with an alternative. "We are still no closer to knowing what actually made them."