Huge diamonds formed near Earth’s core

Rafi Letzter

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Two of the world’s most famous diamonds may have originated super deep below Earth’s surface, close to the planet’s core. All of Earth’s natural diamonds first form deep underground from our perspective on the surface. But from the perspective of this planet’s great bulk, their usual births occur relatively far from the core. Zest the Earth like a lemon and you’d uncover diamonds growing at the bottoms of tectonic plates. Those diamonds form about 150 to 200 kilometres deep under pressure that exists just where the crust meets the more fluid outer mantle, or middle layer of the planet. No mines reach that far underground, but some of those diamonds do make their way up to where humans can reach them. The Hope Diamond, a large and famous stone, as well as the Cullinan Diamond, the largest rough gemstone ever found, are different. They’re ‘super deep’ stones, new research confirms. These boron-blue gemstones likely originated somewhere in the planet’s hot mantle, a region between the crust and the liquid outer core of the planet. This new research shows that, at least sometimes, the stones form deep in this hot zone. The recent research found remnants of a mineral called bridgmanite in two less famous diamonds of the same types as the famous gemstones. All diamonds are crystals made of carbon and various chemical impurities. The type of any specific diamond is determined by the impurities and other conditions present during its creation, so any two diamonds of the same type likely formed in similar conditions. Bridgmanite is a very common mineral inside Earth, but it doesn’t form in the crust or even the upper mantle. “What we actually see in the diamonds when they reach [the] surface is not bridgmanite, but the minerals left when it breaks down as the pressure decreases,” said Evan Smith of the Gemological Institute of America. “Finding these minerals trapped in a diamond means that the diamond itself must have crystallised at a depth where bridgmanite exists, very deep within the Earth.” This discovery suggests both large blue stones originated in the lower mantle, a fluid zone extending from 660 kilometres deep all the way to the planet’s liquid outer core. The first, a 20-carat ‘type IIB blue diamond’ from South Africa, showed evidence of bridgmanite under examination with laser light. The Hope Diamond, at 45.52 carats, is a larger example of the same diamond type. Another diamond, a 124-carat stone about the size of a walnut, is called a ‘CLIPPIR’ diamond, which stands for Cullinan