Diamonds are big business. In 2021 alone, the global diamond industry was worth 87 Billion USD. Diamonds have long captured the imagination with their strength, clarity and association with love. But how are these extraordinary gemstones formed?
We all learned in science class at some point that time and pressure are required to make a diamond. But how does that really work? What forces are at play? And how do those diamonds, buried deep in the ground, come to the surface? New research is helping to answer that last question, but let’s start with a review of the basics.
We’ve all heard the story of a lump of coal turning into a diamond with enough pressure and heat. But as I just learned while reading for this post, that is highly unlikely. Most diamonds have been dated to be older than the source of coal (the first land plants) and coal is sedimentary rocks (sediments that lose the water and become cemented into rock) while the source rocks for diamonds are found in igneous rocks (form when hot, molten rock crystallizes and solidifies).
The process of diamond formation begins in the Earth’s mantle. The mantle is the layer between the Earth’s core and the thin crust. It is about 1802 miles (2900 kilometers) thick. The rocks in the mantle are mostly silicates (composed of silicon and oxygen) and magnesium oxide. Being so big, the mantle does not have a uniform temperature or consistency, with temperature and pressure increasing with the depth of the mantle. The temperatures vary from 1832oF (1000oC) near the crust to 6692oF (3700oC) near the core. The mantle is an active plate, whose activity influences events on the surface. Geologic events such as volcanoes, earthquakes and mountain-building are all the consequence of plate tectonics, which is driven by the mantle.
So if it’s not coal, what is the carbon source for diamond formed in the earth’s mantle? Fascinatingly, a carbon source for diamonds may be the carbon that was trapped in the Earth’s mantle at the time of planet formation. How cool is that? Wearing a diamond is wearing a piece of Earth’s history.
But that’s not the only way diamonds can form. Some diamonds show inclusions (material that is trapped inside a mineral while it is forming) from oceanic crust or sea water. These diamonds are likely formed by subduction. Subduction happens when one of the tectonic plates of the Earth’s crust moves sideways or downward into the mantle beneath another plate. So in this case, oceanic crust was subducted into the mantle, where the conditions were right to make some diamonds.
Lastly, it seems diamonds can also have off-world origins. There have been small (sub-millimeter) diamonds found at asteroid crash sites. The hypothesis being that the high temperature and pressure that an impact creates are sufficient for making diamonds. Diamonds have also been found in some meteorites. These diamonds are exceptionally small (a few nanometers) and are thought to form through high-speed collisions. In some meteorites, up to 3% of the carbon was in nanodiamond form!
For the diamonds formed in the Earth’s mantle, how do they reach the surface? For a long time, geologists and other researchers have known that diamonds appear in a rather violent manner, being blasted to the Earth’s crust through a volcanic eruption. They hitch a ride in kimberlite and lamproite. (Check out this article from GIA about kimberlite as a vehicle for diamonds. It’s a great read. https://www.gia.edu/gems-gemology/summer-2019-kimberlites-earths-diamond-delivery-system )
While scientists have known about the eruptions and worked out the composition of kimberlite and how it acts as a transport for diamonds (without them dissolving completely on the way up), the exact reasons for the eruptions were unknown. Recently, a team at the University of Southampton led by Prof Tom Gernon set up a series of experiments to figure out why. The team analyzed data on the kimberlites and the Earth’s crust and determined that most eruptions happened about 25 million years after the continental plates tore apart. Another pattern emerged from the data, the first eruptions happened near the edges of the plates, with the eruptions moving progressively towards the middle.
Using computer models of rock and magma, the team arrived at a theory. When the continental plates started to drift apart, they stretch and that causes the rock to become thinner. The thinning would then disrupt the normal flow in the Earth’s mantle. The movement of the mantle dislodges chunks of the continental plate that then sink down. These rocks cause even more disruption in the flow of material in the mantle, this time in a wave-like movement that strips layers of rock from the bottom of the continental plates. According to the model, when enough of the melt has formed, it then explodes through the crust in a powerful eruption. This insight into the series of events for diamond formation allows a more targeted approach to finding potential diamond deposits.
While diamonds have long been renowned for their clarity, brilliance and overall beauty, I think they are just as astonishing for their geological complexity. The formation of diamonds not only happens over a long time but likely happened a long time ago. These rare rocks are windows into Earth’s geological history and even occasionally, the geological history of the universe.
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