Earth's diamond days are over

Grains inside diamonds reveal their ages

By Dr David Whitehouse BBC News Online science editor

Geologists have made an unprecedented map of the regions of the Earth where diamonds are formed.

It sheds new light on the process that makes man's most coveted gemstone, and suggests that the Earth no longer makes large diamonds.

"For the first time we have a good map of a diamond forming region," Dr Steven Shirey of the Carnegie Institution in Washington, US, told BBC News Online.

The map was constructed by analysing over 20 years worth of Earthquake data in southern Africa, famed for its diamonds. The speed of pressure waves through the Earth's crust and mantle provided clues about the rock composition in the diamond forming region.

Parts of an ancient sea bed found inside a diamond

The data were combined with an analysis of impurities in over 4,000 diamonds that provided their ages as well as details of the rocks in which they formed. All but a few are billions of years old.

Splinters from fallen stars

The ancient Romans believed diamonds were splinters from fallen stars. The Ancient Greeks thought they were tears from the Gods.

Most civilisations have had a fascination for them, if not for their beauty and rarity then for their useful properties - they are the hardest substance known.

Today we know that diamonds form at great pressures (typically 50,000 atmospheric pressures) and at great depths (about 200 km) from carbon deposits compressed so much that the carbon atoms form a crystalline lattice. After they have been formed they are brought to the surface in magma from volcanoes.

But much about the formation of diamonds remains mysterious. From the data they obtained, geologists have now identified three generations of diamonds.

The first generation was formed about 3.3 billion years ago, Steve Shirey told BBC News Online. "The oldest diamonds are survivors from the Earth's earliest geological times and the oldest known rocks. We were able to identify where they came from under southern Africa."

The second generation of diamonds were formed slightly later, about 2.9 billion years ago. These appear to be more widely scattered throughout a bigger region of the Earth's mantle. These diamonds may have been formed in a slightly different way from their first-generation cousins.

From an analysis of impurities in them scientists believe they formed inside rocks that were originally laid down in an ancient shallow sea. Somehow these rocks were pulled down to great depths where carbon deposits, possibly from living organisms, were heated and compressed to form diamonds.

The third generation of diamonds are termed Proteozic by some scientists. They formed about 1.2 billion years ago and provide a particular insight into the conditions on Earth at a geologically significant time.

End of an era

"I think of diamonds as being tiny time capsules that encase a little piece of rock protecting it for billions of years and providing us with a unique window on ancient times," Steve Shirey said.

Some younger diamonds - about 100 million years old - are known but they are a minority and not fully understood.

However these youngsters were formed, it seems that the era of diamond formation is over.

"We believe that the Earth is not forming as many or as big diamonds as it did billions of years ago," says Dr Shirey.

"Something was different then. Perhaps the planet was hotter on the inside, or the composition of the rocks was subtly different. Whatever it was it has changed now. Diamond formation was chiefly a feature of the Earth's youth."

The research is published in the journal Science and is a collaborative project among many universities and institutions.