The gemstone was genuine. It would come to bear the name of the mine’s owner, Sir Thomas Cullinan, who mailed it to London (costing, Century Magazine noted in 1909, $1 in postage while insured at a value of $1.25 million). There, the government of the South African colony Transvaal presented it to British monarch King Edward VII for his birthday. Jewelers carved the Cullinan diamond into nine principal stones — the largest two remain among the Crown Jewels — and dozens of smaller gems.
Most diamonds, of course, do not weigh almost as much as a regulation NBA basketball. But a very few of the carbon crystals, which have earned names such as the Cullinan, the Constellation diamond and the Koh-i-Noor, are far larger than the average engagement gem.
To geochemists like Evan M. Smith, at the Gemological Institute of America in New York, the stones have more than monetary worth or pretty sparkles. The material, and its imperfections, are valuable. Diamonds are hardened capsules of chemical information, containing insights into forces hundreds of miles below the earth. As Smith told NPR, “Diamond is the ultimate Tupperware.”
Smith and his colleagues at American, Italian and South African research institutions recently published an examination of these stones in the journal Science. The typical diamond formed about 100 miles beneath the surface, where pressure squeezed pockets of carbon atoms into precious crystals. The giant diamonds, the new research suggests, were birthed in liquid metal pools even deeper below.
For chemical analysis, the scientists collected a handful of offcuts — the scraps and shavings that result from crafting jewels — from some of the largest diamonds. Such diamonds, like the Cullinan, have little nitrogen content. They tend to be lumpy or irregular in shape and have very few of the flaws called inclusions.
But these diamonds are not completely perfect. Thanks to an analysis of the inclusions within the offcuts, as well as an inspection of 53 Type II diamonds, which are free of nitrogen, the researchers discovered globs of trapped metal. Three in four diamonds contained iron and nickel alloys in their imperfections, plus sulfur, carbon and hydrogen compounds.
The scientists also detected “a thin fluid jacket” of methane, the authors wrote in the study, that surrounded the inclusions like a film. Fifteen of the diamonds, too, had traces of the mineral garnet.
Taking all of the chemical clues together, the inclusions suggested the existence of liquid metal pockets in Earth’s rocky mantle between 240 and 460 miles below the surface. (Garnet is unstable beyond 466 miles beneath the surface, the scientists noted.) That is as far below our feet as satellites in low Earth orbit, NPR noted, are above our heads; the Kola Superdeep Borehole, the world’s deepest human-made hole, goes down 7.5 miles beneath Russian soil.
After the diamonds crystallized out of the liquid metal, shafts of erupting rock called kimberlite propelled the gems to the surface. The kimberlite pipes may have traveled violently, thrust upward at speeds of 30 miles an hour, National Museum of Natural History geologist Jeffrey E. Post told Smithsonian Magazine in 2006. “Once the diamonds are brought to the surface and cooled relatively quickly, those carbon atoms are locked into place,” he said, preventing the atoms from forming graphite, another all-carbon structure.
Earlier experiments hinted at metallic iron in the deep mantle. Smith, in a news release, called the Type II diamond inclusions “consistent, systematic physical evidence to support this prediction.”
Researchers are examining inclusions in billions-year-old diamonds to learn not just about the deep Earth but also the planet’s ancient history. Steven Shirey, a co-author of the recent study and a geochemist at the Carnegie Institution for Science, said in a 2011 study release that diamond imperfections can “provide age and chemical information for a span of more than 3.5 billion years that includes the evolution of the atmosphere, the growth of the continental crust, and the beginning of plate tectonics.”
More from Morning Mix