Scientists think that most of the materials deep below the surface of the Earth have high distribution of oxygen. Findings of a new study that involved analysis of the world's biggest diamonds, however, revealed something else.

Diamond Inclusions

Diamonds are formed deep below the crust in the mantle and are only brought to the Earth's surface during volcanic eruptions.

The precious stones come with tiny flecks of metals and minerals trapped inside them, which are the so called "inclusions" that are removed by jewelers to polish the gemstones when they are sold as jewels.

In a new study, scientists analyzed diamond inclusions to glean information on the composition of the Earth's interior.

Evan Smith of the Gemological Institute of America and colleagues looked closer at the scraps left over from polishing and cutting type IIa diamonds, the variety of big and expensive diamonds to which popular stones such as Koh-i-Noor, Lesedi La Rona, Constellation and Cullinan belong.

The analysis, which was reported in the study published in the journal Science on Dec. 15, revealed that the inclusions contain iron, nickel, carbon and sulfur that are encased in a layer of liquid methane and hydrogen.

Where The Biggest And Most Expensive Diamonds Are Formed

The metallic inclusions suggest that the stones formed under extreme pressure in oxygen-deprived patches of liquid metal.

Mineral inclusions in some of the large diamonds also hint that large and expensive diamonds are born at much greater depth compared with the smaller ones.

Cullinan-like gems likely formed at depths ranging within 360 to 750 km (about 224 to 466 miles) in the convecting mantle, which is far deeper than where most other gem diamonds form at 150 to 200 km (about 93 to 124 miles) in the lower part of continental tectonic plates.

What The Earth's Mantle Is Composed Of

Researchers earlier believed that the Earth's mantle has a uniform mix of oxygen-rich rocks, but the study of the large diamonds that were formed in oxygen-deprived patches hinted that such theory may not be entirely correct.

Smith said that previous experiments and theory have predicted that the region of the deep mantle below 250 kilometers (155 miles) have small amounts of metallic iron and contain limited amounts of oxygen. He said that their analysis offers evidence supporting this prediction.

"We verify previous predictions that Earth has highly reducing deep mantle regions capable of precipitating a metallic iron phase that contains dissolved carbon and hydrogen," the researchers reported.