✦ These 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, Emma Bullock, and Jianhua Wang shows them to crystallize in the mantle transition zone (440-660 km deep) in native iron metal bearing regions characteristic of the deep mantle. These reducing regions, long hypothesized but always hidden by melting processes in shallower and more oxidized mantle, had never been sampled before this research. 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.