'Finest quality diamonds form deeper in Earth'

Blue diamonds - like the world-famous Hope Diamond - formed up to four times deeper in the Earth's mantle than most other gems, according to a study.

"These so-called type IIb diamonds are tremendously valuable, making them hard to get access to for scientific research purposes," said Evan Smith from the Gemological Institute of America.

"It is very rare to find one that contains inclusions, which are tiny mineral crystals trapped inside the diamond," said Smith, lead author of the study published in the journal Nature.

Inclusions are remnants of the minerals from the rock in which the diamond crystallised and can tell scientists about the conditions under which it formed.

Type IIb diamonds owe their blue colour to the element boron, an element that is mostly found on the Earth's surface.

The Hope Diamond is one of the most famous jewels in the world. Its much-admired rare blue colour is due to trace amounts of boron atoms.

However, analysis of the trapped mineral grains in 46 blue diamonds examined over two years indicate that they crystallised in rocks that only exist under the extreme pressure and temperature conditions of the Earth's lower mantle.

The team, including researchers from the Carnegie Institution for Science (CIS) in the US, determined that blue diamonds form at least as deep as the transition zone between the upper and lower mantle - or between 410 and 660 kilometers below the surface.

Several of the samples even showed clear evidence that they came from deeper than 660 kilometers, meaning they originated in the lower mantle.

By contrast, most other gem diamonds come up from between 150 and 200 kilometers.

According to a hypothesis put forth by the research group, the boron came from seafloor that was conveyed down into the Earth's mantle when one tectonic plate slid beneath another - a process known as subduction.

The study proposes that boron from the Earth's surface was incorporated into water-rich minerals like serpentine, which crystallised during geochemical reactions between seawater and the rocks of the oceanic plate.

This reaction between rock and water is a process called serpentinisation and can extend deep into the seafloor, even into the oceanic plate's mantle portion.

The discovery reveals that the water-bearing minerals travel far deeper into the mantle than previously thought, which indicates the possibility of a super-deep hydrological cycle.

"Most previous studies of super-deep diamonds had been carried out on diamonds of low quality," said Carnegie's Steven Shirey.

"This new discovery that blue diamonds also have super-deep origins, we now know that the finest gem-quality diamonds come from the farthest down in our planet," Shirey said.