The Very Deep Origin of the World's Biggest Diamonds

gemstones for jewelry, are some of the most scientifically valuable samples of the deep earth. Interpretation of their included minerals and crystallization history provides a snapshot of otherwise inaccessible geological processes from more than 100 kilometers underground and often from billions of years in the past (e.g., Shirey and Shigley, 2013). No other mineral sample can provide information for research from such depths and from so long ago. Until recently, exceptional gem diamonds such as the famous Cullinan or Lesotho Promise, with a set of physical characteristics that distinguish them from other kinds of diamonds, were enigmatic and had an unknown origin (Bowen et al., 2009; Gurney and Helmstaedt, 2012; Moore, 2014). How did they form? Certainly not like 99% of other gem diamonds. New research carried out in the last two years at GIA, the Carnegie Institution for Science, and the University of Padua reveals that these Cullinan-like diamonds form in a geologically special way, involving metallic liquid and originating from extreme depths in Earth’s mantle well below that of other gem diamonds (Smith et al., 2016).