COAUTHORS AND SUPPORT: Other coauthors on the paper are Stephen Richardson of University of Cape Town, Fabrizio Nestola of University of Padua, and Wuyi Wang of the Gemological Institute of America. A Gemological Institute of America Liddicoat Postdoctoral Research Fellowship, the Deep Carbon Observatory, and the European Research Council supported this research. lue diamonds, like the world-famous Hope Diamond, formed up to four times deeper in the Earth’s mantle than most other diamonds, according to new work published on the cover of Nature. The discovery received wide media attention. The research group, including Carnegie’s Steven Shirey, Emma Bullock, and Jianhua Wang, determined that blue diamonds form at least as deep as the transition zone between the upper and lower mantle, or between 255 and 410 miles (410 and 660 kilometers) below the surface. Several of the samples even showed clear evidence that they came from deeper than 410 miles, meaning they originated in the lower mantle. By contrast, most other gem diamonds come up from between 93 and 124 miles (150 and 200 km). “These so-called type IIb diamonds are tremendously valuable, making them hard to get access to for scientific research purposes,” explained lead author Evan Smith of the Gemological Institute of America, adding, “and it is very rare to find one that contains inclusions, which are tiny mineral crystals trapped inside the diamond.” Inclusions are remnants of the minerals from the rock in which the diamond crystallized; they can tell scientists about the conditions under which the diamond formed. Type IIb diamonds owe their blue color to the element boron, which is mostly found on the Earth’s surface. But analysis of the trapped mineral grains in 46 blue diamonds examined over two years indicate that they crystallized in rocks that exist only under the extreme pressure and temperature conditions of the Earth’s lower mantle. So how did the boron get down there if it is an element known for residing predominately in the shallow crust? According to the group’s hypothesis, it 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 new study proposes that boron from the Earth’s surface was incorporated into water-rich minerals like serpentine, which crystallized during geochemical reactions between seawater and the rocks of the oceanic plate. This reaction between rock and water is a process called serpentinization and can extend deep into the seafloor, even into the oceanic plate’s mantle portion. The group’s 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,” Shirey said. “But between our 2016 finding that the world’s biggest and most-valuable colorless diamonds formed from metallic liquid deep inside Earth’s mantle and 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.”  What Makes Diamonds Blue? The August cover of the journal Nature featured the paper revealing the super-deep origin of blue diamonds, on which Evan Smith of the Gemological Institute of America is lead author and Carnegie’s Steven Shirey (middle), Emma Bullock (left), and Jianhua Wang (right) are coauthors.  This photo shows a blue, boron- bearing diamond with dark fragments of a mineral called ferropericlase, which were examined as part of this study. This gem weighs 0.03 carats. Image courtesy Evan Smith/ Gemological Institute of America  This larger, blue, boron-bearing diamond contains mineral fragments that were examined as part of this study. This gem weighs 3.81 carats and is 1.26 centimeters long. Image courtesy Robison McMurtry/ Gemological Institute of America Carnegie Science | Fall 2018 6  