'World's biggest, best diamonds formed deep inside Earth's mantle'

New York: The world's biggest and most-valuable diamonds like the famed Kohinoor may have formed from metallic liquid deep inside the Earth's mantle, according to a new study.

Researchers from the Gemological Institute of America studied the unique properties of diamonds with similar characteristics to famous stones such as the Cullinan, Constellation and Kohinoor by examining their so-called "offcuts," which are the pieces left over after the gem's facets are cut for maximum sparkle.

They determined that these diamonds sometimes have tiny metallic grains trapped inside them that are made up of a mixture of metallic iron and nickel, along with carbon, sulphur, methane and hydrogen.

These inclusions indicate that the diamonds formed, like all diamonds, in the Earth's mantle, but they did so under conditions in which they were saturated by liquid metal.

As unlikely as it sounds, their research shows that pure carbon crystallised from this pool of liquid metal in order to form the large gem diamonds.

"The existence of this metal mixture has broad implications for our understanding of deep Earth processes," said Evan Smith from the Gemological Institute of America.

Diamonds form deep in the Earth's mantle and shoot to the surface in minor volcanic eruptions of magma.

Impurities contained inside diamonds can teach geologists about deep Earth chemistry under the pressure, temperature, and chemical conditions in which they were formed.

Diamonds, once formed, have a unique ability to protect and shield any minerals contained inside their crystal structures, thereby giving scientists a special, protected sample of the mantle mineralogy and a glimpse at conditions miles beneath the planet's surface.

Most diamonds form at depths around 145 to 241 kilometres under the continents. However, the so-called "superdeep" diamonds form much deeper - at depths below 386 kilometres, where the mantle rocks are known to be mobile due to convection.

From the team's work, we now understand for the first time that large gem diamonds are a group of superdeep diamonds, according to analysis of tiny samples of silicate that were also found inside the studied diamonds.

These tiny silicate inclusions are also associated with the metal. These tiny samples of metal, along with their associated methane and hydrogen, tells researchers about oxygen availability in different parts of the mantle.

Near the surface, the mantle chemistry is more oxidised, which scientists can tell from the presence of carbon in the form of carbon dioxide in magmas erupted in volcanoes.

However deeper down, according to the researchers, some regions of the mantle are the opposite of oxidised, or reduced, which is what allows the iron-nickel liquid metal to form there.

The study was published in the journal Science.