Geologists have long pondered how and where the largest and most expensive diamonds found on Earth were formed. Most ‘normal’ sized diamonds are thought to have formed in the lower part of continental tectonic plates at depths of between 93-124 miles (150-200 km) below the surface. But the largest diamonds display slightly different characteristics than these and now Dr. Wuyi Wang, director of research and development at the Gemological Institute of America has shown that these stones formed much, much deeper into the Earth’s mantle (between 224-466 miles (360-750 km)) from metallic liquid in the convecting layer. It has always been difficult to study these diamonds due mainly to their immense value but Dr. Wuyi Wang and his team used the ‘off-cuts’ from these exceptional gems (the tiny fragments that have been cut or polished off to make the diamond have maximum sparkle!) to study their chemical make-up and make new discoveries about their origin.
The researchers discovered that these large diamonds contained tiny metallic crystal grains trapped inside their structures along with minerals. The metallic grains (known as inclusions) were found to contain a mixture of the elements Iron, Nickel, Carbon and Sulfur, all of which are found in the Earth’s mantle in liquid form. More surprisingly, in the tiny gaps between the between the metallic grains and the rest of the diamond they found traces of fluid methane and Hydrogen, trapped as the diamond solidified. These elements are only found in this form in the deeper part of the mantle and so give proof to the origin of these amazing gemstones and so support Dr. Wang’s theory. He said, “Now, the metallic inclusions and their surrounding methane and hydrogen jackets in these diamonds provide consistent, systematic physical evidence to support this prediction.”