Qld scientists unearth new theory on origin of diamonds

When QUT professor Balz Kamber took a sledgehammer to a chunk of rock at a South African diamond mine, he found something that would end up being more valuable than the precious stones themselves – the key to how they form.

Kamber was rummaging through an old pile of waste rocks looking for one that contained a distinctive purple crystal called garnet harzburgite.

Researcher Carl Walsh (right) with Professor Balz Kamber and a sample of the type of rock used in their modelling.Credit: QUT

The rocks were mined anywhere between 1871 and 1914 but were themselves far older, with the one Kamber broke off believed to be 3.3 million years old.

Using this rock and others like it as a guide, Kamber and his colleagues have done fresh computer modelling that shows diamonds are rarer than previously thought.

Existing models of diamond creation suggest they are formed inside host rocks that also form these purple garnets, and that there used to be a lot of them but many were destroyed by huge tectonic forces over time.

However, Kamber said their new modelling suggested the diamonds were not destroyed, but that far fewer formed than previously thought.

“It was thought that diamonds were forming at the base of the continental plates millions of years ago, but that the moving and grinding of these plates would have destroyed many of the diamonds,” he said.

“By contrast, we suggest that diamond has always been rare – it only rarely formed, and actually survived to this day.”

QUT PhD candidate Carl Walsh, the lead research author, said they used the composition of the rock that Kamber had sourced and used it to create a new computer model for how diamonds were formed.

“We basically had a known starting composition of a rock, which is representative of Earth’s mantle at an early time in the history of the Earth, before all the continents were formed,” Walsh said.

“We took that starting composition and modelled what would happen to it if it was progressively melted, and what would be left over.”

The modelling suggested the specific conditions that produced the purple garnet would also produce host rocks ideal for filtering carbon out of the surrounding material and turning it into diamond.

This also goes against the previous theory that temperatures over a certain level would produce graphite.

Instead, the researchers believe their model shows that the high temperatures would have produced the ideal host rocks, and that diamonds would have formed inside them later as carbon was filtered and compressed.

Kamber said the findings had implications for diamond mining, giving miners a better sense of where to look for new deposits.

And he said it also gave a fresh perspective for people who already owned a diamond.

“Whoever has a diamond on their finger now should be even more appreciative of how rare diamonds have always been,” he said.

The research has been published in the journal Nature.