Large diamonds, known by scientists as type II diamonds, were discovered to be formed near the center of  Earth. A new study analyzed imperfections trapped inside the rocks and found that they were forged within blobs of liquid metal hundreds of kilometers deep underground.

Studying diamonds, especially big ones, is a difficult task because people is more interested in using them as jewelry than an examination object. Scientists from the Gemological Institute of America (GIA) in New York City had the chance to handle a significant number of large diamonds to study their origins. The institute often processes thousands of gems each day and was able to work with 53 type II diamonds.

Type II diamond. Image credit: Nemesis International DMCC/ Donald Woodrow photographer.

The team focused on studying the large rocks’ inclusions, which are small chunks of material trapped inside. Those pieces of evidence revealed that 38 of the 53 samples contained a graphite-coated mix of metal-rich minerals. The inclusions also had an alloy of iron and nickel along with hydrogen and methane.

The findings suggest that the diamond’s imperfections were once a hot mix of iron, nickel, carbon-sulfur and other trace elements. Those minerals are found deep underground, which means the large diamonds were originated near the Earth’s core.

The inclusions in the remaining valuable stones contained silicate minerals such as garnet. Scientists estimate that the valuable rocks could have formed at depths between 360 and 750 kilometers under the surface, where minerals are not stable elements.

The study was published Thursday in the journal Science. Its findings are crucial to understanding how volatile substances like carbon behaves through Earth’s interior over time.

Bubbling liquid metal leaves its trace on large diamonds

Diamonds –after being formed deep underground— reached the surface in sudden eruptions through processes scientists still do not understand. Those eruptions leave traces of tubular deposits –kimberlites– which are the primary source of most gems on Earth.

The type II diamonds included in the study are a door to the origins of our planet. The inclusions are trapped inside the gems, and physical and chemical information gets preserved thanks to the isolation of the diamonds. The problem is that because many of them fall into jeweler’s hands, the inclusions are cut away because they are seen as imperfections in the stone.

“The diamonds have delivered these well-preserved materials to us at the surface,” said study co-author Steven Shirey, a geochemist at the Carnegie Institution for Science in Washington, D.C. “They’re a classic example of how the tiniest bits of material can tell us big things about our planet.”

The researchers only focused on type II diamonds. Thus, the study findings do not apply to the origin of all gems, although the results provide insights into how other diamonds are created.

Evan Smith, a geologist at the GIA and co-author of the study, said that the mantle is abundant in iron-rich minerals under high pressure, meaning that large quantities of diamonds are waiting to be blasted to the surface in gem-studded eruptions.

Graham Pearson, a geochemist at the University of Alberta in Edmonton, Canada, stated that the study goes a “long way” when it comes to explaining type II diamonds formation.

Source: Science