They are made of the element that is the basis of life itself: carbon. They were formed billions of years ago. Diamonds dazzle us, literally and poetically speaking. They form naturally in a few places on Earth. For example, in the depths of continental cratons or in the impact of a meteorite. And among these, there is one that is more interesting than the rest. They are the enigmatic blue diamonds.
The telltale imperfections
They reach the surface in an explosive way, in the magma of some of the strangest eruptions in history. Those that have the few volcanoes that have their roots in the deepest part of the planet.
All diamonds, once formed, have a unique ability. They harbor and protect any minerals contained within their crystalline structures. It gives scientists a special glimpse into the mineralogy of the mantle. It is a clear glimpse into conditions miles below the planet’s surface. And in that sense, the blue diamond is exceptional.
Most diamonds form at depths of about 150 kilometers beneath the continents. Blue ones originate up to four times deeper, in the Earth’s lower mantle.
They are valuable as they are usually very pure. They tend to have no “inclusions,” or small bits of non-diamond material. Those imperfections give scientists more information. They were able to analyze 46 blue diamonds with inclusions.
Several of the samples even showed clear evidence that they came from more than 660 kilometers away. They originated in the lower mantle. That makes them true time capsules.
“We can’t get to the interior of the Earth. Diamonds form down there and usually encapsulate whatever is down there.” Says Reel George Harlow, a geologist at the American Museum of Natural History in New York.
The origin of blue
“They’re like our space probes. Eventually, some make it to the surface of the Earth so we can study them.”
They were known to be blue because they contained trace amounts of boron, a metalloid chemical element. But blue diamonds form in the Earth’s mantle. And boron is concentrated in the crust. So where did these diamonds get their boron?
One theory is that the boron came from the seafloor. And it was transported into the Earth’s mantle when one tectonic plate slid under another. It spread deep into the seafloor, even into the mantle portion of the oceanic plate.
The enigmatic blue diamonds hold even more secrets. In their crystalline structure, millions of years of secrets are preserved. As Harlow says, blue diamonds “are not only beautiful and rare, but extremely interesting. They teach us a lot about our planet.