A new analysis of inclusions, or the small imperfections, trapped within the biggest and most prized diamonds offer evidence that these gemstones were formed within pools of liquid metal deep below the Earth's surface.
Scientifically Valuable Gemstones
The biggest diamonds on Earth are prized gemstones. They also happen to be scientifically valuable because they come from a deep part of the Earth that humans do not know much about.
Scientists have difficulty getting their hands on these precious stones but a team of scientists have come up with a way to conduct a study of a subset of type II diamonds, the kind with low nitrogen content that makes them very clear.
Pure Carbon Crystallized From Liquid Metal
Jewelers carve diamonds to enhance their shine and the scraps are called offcuts. Gemological Institute of America (GIA) geologist Evan Smith and colleagues examined the offcuts of large and valuable diamonds and found that diamonds sometimes have tiny metallic grains that are trapped inside them.
These grains are composed of a mixture of metallic iron, carbon, nickel, sulfur, methane and hydrogen suggesting that the largest gem diamonds are pure carbon crystallized from liquid metal.
Just like all diamonds, these large diamonds form in the Earth's mantle. What makes this type of diamonds different is that they were formed under conditions in which they were saturated by liquid metal. The research suggests that pure carbon crystallized from this pool of liquid metal to form large gem diamonds.
"Large, exceptional gem diamonds like the Cullinan, Constellation, and Koh-i-Noor carry direct evidence of crystallization from a redox-sensitive metallic liquid phase in the deep mantle," the researchers wrote in their study.
"The metal-dominated mineral assemblages and reduced volatiles in large gem diamonds indicate formation under metal-saturated conditions."
Besides finding tiny metallic fragments, the researchers also found tiny pieces of silicate trapped in offcuts. The tiny silicate inclusions are associated with metal and the composition hint the gem diamonds were formed much deeper than average beneath the Earth's surface.
Most diamonds were formed about 90 to 150 miles below the surface but the so-called superdeep diamonds form at depths reaching 240 miles, where the Earth's mantle rocks are known to be mobile because of convection. The depth is about twice as deep as where diamonds are commonly born.
Based on analysis of the tiny samples of silicate inside the studied diamonds, the researchers concluded that large gem diamonds belong to the group of superdeep diamonds.