Scott Johnson, a geologist and "Ars Technica" contributor, recently wrote that “[c]ontrary to popular culture, diamonds are not formed from the metamorphosis of coal under tremendous heat and pressure. It makes for nice poetry, but it’s not true.”
Indeed, scientists know quite a bit about how diamonds form. It’s a fascinating tale of heat, violence and high pressure; and it also gives new insights into how Earth and other planets formed.
♦ Diamond facts
The chemical formula for diamond is simply C, because this mineral is pure carbon. No other gem surpasses its beauty when it has been shaped and polished for diamond rings, but according to experts, diamonds also are the hardest known natural substance, coming in at 10 on the Mohs scale. They also have the highest melting point of any substance at well over 7000 degrees Fahrenheit or over 4000 degrees Celsius. If you touch a diamond at room temperature, it will feel cold to you because the stone is actually conducting heat out of your hand: diamond has the greatest heat conductivity of any natural mineral.
In addition to these extreme physical qualities, diamonds usually arrive in a spectacular way, out of great depths via rare supersonic eruptions of a kimberlite or lamproite magma. Diamonds can also be quite old. The diamond in your jewelry may have formed some 3 billion years ago; to put that into perspective, geoscientists believe that the Earth itself is around 4.5 billion years old!
♦ How diamonds form on Earth
Their great age and a source deep within the planet, along with their usually being found in very old rocks, called cratons, at the cores of continents, all suggest that diamonds form underneath continents in what geologists call the cratonic keels.
While "cratonic keels" would be a great name for a rock group, it's also an apt analogy. When a ship is built, the first thing they do is lay the keel - a long structural beam around which the rest of the ship’s hull is built. Continents don’t exactly sail around over a molten mantle in plate tectonics theory, but they do move over the surface of the Earth. Geologists were both surprised and puzzled to discover, that at their cores, all continents have a thick layer of strong mantle underneath that moves with them.
This cratonic keel layer has carbon and many other elements in it, just as at Earth’s surface, but it is very hot and also is under tremendous pressure. Carbon forms graphite (pencil lead) at surface temperatures and pressures, but in the extreme conditions underneath moving continents, carbon is stable only in the very tightly bonded internal lattice structure of what we call diamonds when they are blown to the surface in a kimberlite eruption many millions of years later.
♦ Diamond in space
Not all diamond-like minerals can be explained by the cratonic keel theory. A polycrystalline type of diamond called carbonado is only found in Brazil and at one location in Africa, and is older than Earth. Some scientists think carbonado diamonds may have formed in a star, perhaps a supernova, but so far no one is really sure about their origin.
It may also be raining diamonds on the gas giant planets of our solar system that have a lot of methane in their atmosphere. Methane contains carbon, and when a team of geoscientists at UC Berkeley applied intense pressure and heat to some methane they had placed in a diamond anvil cell, it formed diamond dust. The researchers believe that the process of turning methane into diamonds could explain why Neptune radiates more heat than it receives from the Sun.
Here on Earth, diamonds may not form from coal, and they certainly don't rain from the sky, but they are a scientist’s best friend when it comes to understanding how our planet works and what may be going on elsewhere in our solar system.