Sydney, May 30 (IANS): Diamonds are mostly formed from ancient seabeds that became buried deep beneath the Earth's crust, says a study that found traces of salt trapped in many of the stones.

Most diamonds found at the Earth's surface are formed this way, while others are created by crystallisation of melts deep in the mantle, said the study published in the journal Science Advances.

In experiments recreating the extreme pressures and temperatures found 200 kilometres underground, researchers have demonstrated that seawater in sediment from the bottom of the ocean reacts in the right way to produce the balance of salts found in diamonds.

The study settles a long-standing question about the formation of diamonds.

"There was a theory that the salts trapped inside diamonds came from marine seawater, but couldn't be tested," said lead author Michael Forster from Macquarie University in Sydney, Australia.

"Our research showed that they came from marine sediment."

Diamonds are crystals of carbon that form beneath the Earth's crust in very old parts of the mantle. They are brought to the surface in volcanic eruptions of a special kind of magma called kimberlite.

While gem diamonds are usually made of pure carbon, so-called fibrous diamonds, which are cloudy and less appealing to jewellers, often include small traces of sodium, potassium and other minerals that reveal information about the environment where they formed.

These fibrous diamonds are commonly ground down and used in technical applications like drill bits.

Fibrous diamonds grow more quickly than gem diamonds, which means they trap tiny samples of fluids around them while they form.

"We knew that some sort of salty fluid must be around while the diamonds are growing, and now we have confirmed that marine sediment fits the bill," Forster said.

"We demonstrated that the processes that lead to diamond growth are driven by the recycling of oceanic sediments in subduction zones," Forster added.