Experts judge a diamond’s worth by the four Cs, “color, clarity, cut, and carat weight,” and blue diamonds are no exception. These rare and expensive gemstones have a sparkling blue color and exceptional clarity. The largest ones measure more than 100 carats in their rough state. Scientists have puzzled over how these unusual diamonds form, and have finally come up with a fifth word that starts with “C” to describe their origins: crust.
DCO researchers have found evidence that blue diamonds come from an exceptionally deep source, from the lower mantle, and that they form from materials that originated in the oceanic crust. Members of the Extreme Physics and Chemistry Community and the Reservoirs and Fluxes Community, Evan Smith (Gemological Institute of America (GIA), USA), Steve Shirey (Carnegie Institution of Washington, USA), Fabrizio Nestola (University of Padua, Italy), and colleagues, report these findings in a new paper in the journal Nature .
“This is the first real model for how you get a blue diamond,” said Shirey. “Before this, no one knew how these blue diamonds form.”
Scientists have long known that blue diamonds get their color from trace amounts of boron that replace carbon in the crystal structure, but the source of that boron was a mystery. “Diamonds come from the mantle, and we know that the mantle is a barren place for boron,” said Smith. Boron tends to stay on Earth’s surface, concentrated in seawater and the crust.
Getting blue diamonds, like the famous walnut-sized Hope diamond, into the lab to study them, however is a difficult proposition. “It’s hard to get samples,” said Smith. “You can’t borrow the crown jewels, and you can’t buy samples.” Smith, as a research scientist at GIA, was in a unique position to examine the very few blue diamonds among the millions of diamonds that pass through GIA’s labs each year for grading and analysis.
At GIA, Smith examined 46 blue diamonds, including the Cullinan Dream, which at 24 carats is about the size of a grape. Specifically, he looked for tiny mineral grains, called inclusions, which become trapped in the diamond as it forms, and which give hints to where the diamond originated. The researchers primarily relied on a non-destructive technique called Raman spectroscopy to identify the minerals within the inclusions. This technique involves shining a laser at the sample and measuring the unique spectrum of light scattered by each mineral contained within.
The inclusions from all of the blue diamonds contained groups of minerals not usually found within typical white diamonds that form at shallower depths. For example, many of the minerals detected in the inclusions are what you would expect to see in ocean crust after it has been transformed by intense heat and pressure from subduction, when one piece of tectonic plate sinks beneath another and dives into the mantle below. “When you piece them all together, you get a mineral assemblage that shows the diamonds crystallized in an oceanic plate after the plate has been subducted down to the lower mantle,” said Smith.
These findings also can explain how blue diamonds get their boron. The researchers propose that pieces of ocean crust and the underlying mantle accumulated boron from percolating seawater, through a chemical reaction called serpentinization, before being subducted into the mantle.
The researchers also found further evidence of blue diamonds’ watery past. They noticed thin “jackets” of methane and hydrogen surrounding the inclusions in some of the diamonds. The jacket suggests that the minerals had excess hydrogen, which may have come from seawater via serpentinization. This hydrogen escaped from the minerals in the inclusion during the diamond’s trip back up to the surface. The hydrogen was still trapped inside the diamond, however, where it reacted with the diamond’s carbon to create methane.
“This may be some of the first evidence we have to show that water can be incorporated into the ocean plate and survive subduction all the way down to the lower mantle,” said Smith.
In future work, the researchers hope to test out this idea by acquiring blue diamonds that they can examine with less gentle methods. By investigating the mix of boron isotopes in the diamonds, they hope to find further clues about where the boron originated. They also would like to analyze the minerals inside the inclusions more thoroughly to determine the ages of these diamonds.
“Diamonds are not just for beautiful jewelry, they’re also some of the most scientifically valuable pieces of our Earth,” said Smith. “Diamonds offer a way of seeing what happens deep inside Earth, over billion-year time scales. No other mineral can do that.”
Main image: This blue boron-bearing diamond weighs 0.03 carats and has dark inclusions of a mineral called ferropericlase, which were examined as part of the study. Credit: Evan M. Smith/GIA