WASHINGTON - ?Scientists claimed they have figured out how the world's biggest and most-valuable diamonds formed.

In a study published this week in the US journal Science, they said large gem-quality diamonds, like the world-famous Cullinan or Lesotho Promise, may be born in metallic liquid deep inside Earth's mantle.

The research team, led by Evan Smith of the Gemological Institute of America, reached the conclusion after examining so-called "offcuts" of massive diamonds, which are the pieces left over after the gem's facets are cut for maximum sparkle.

They found tiny metallic grains trapped inside in more than 30 exceptionally large stones, which are made up of a mixture of metallic iron and nickel, along with carbon, sulfur, methane, and hydrogen.

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

Most diamonds formed at depths of 150 to 200 kilometers under the continents and shoot to the surface in volcanic eruptions. But these large, rare stones formed at extreme depths, likely within 360 to 750 kilometers in the convecting mantle, where rocks are known to be mobile.

"Pure carbon crystallized in this mix of molten metallic liquid in Earth's deep mantle to form diamonds," the team explained in a statement.

"Small droplets of this metallic liquid were occasionally trapped within the diamonds as they grew," offering useful clues that may help "advance our understanding of Earth's deep mantle, hidden beneath tectonic plates and largely inaccessible for scientific observation."

The findings may tell scientists about oxygen availability in different parts of the mantle.

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

But deeper down, according to the team's findings, some regions of the mantle are the opposite of oxidized, or reduced, which is what allows the iron-nickel liquid metal to form there.

"Previous experiments and theory predicted for many years that parts of the deep mantle below about 250 kilometers depth contain small amounts of metallic iron and have limited available oxygen," Smith said.

"Now, the metallic inclusions and their surrounding methane and hydrogen jackets in these diamonds provide consistent, systematic physical evidence to support this prediction."