Steven B Shirey
Senior Staff Scientist
Earth and Planets Laboratory
Carnegie Institution for Science
✦ These types of rough diamonds, besides being the world's most valuable gemstones based on size and clarity, are remarkable for geological reasons. Research done at the Gemological Institute of America by Evan Smith and the Carnegie Institution for Science by Steve Shirey, Peng Ni, Anat Shahar, Emma Bullock, and Jianhua Wang shows them to crystallize in the mantle transition zone (440-660 km deep) and to carry metallic inclusions. The iron in the inclusions was derived from magnetite and awaruite crystallized during serpentinization of peridotite on the ocean floor before being carried to great depth by subduction. See: Smith, E.M., Peng, N., Shirey, S.B., Richardson, SH., Wang, W., and Shahar, A. (2021) Heavy iron in large gem diamonds traces deep subduction of serpentinized ocean floor. Science Advances 7: eabe9773 https://doi.org/10.1126/sciadv.abe9773
Photo above is by Robert Weldon. © 2015 GIA. Courtesy of Gem Diamonds Ltd. This group of type IIa rough diamonds (VRL# 190894) shown here range from 14 to 91 carats and sometimes appear to be broken fragments of once larger diamonds.