To fully appreciate the journey a diamond goes through to end up in a finished item of jewellery, it is best to understand how diamonds are formed. Many people know that diamonds are made up of carbon, but beyond that, what is it that makes diamonds so special and so rare? To answer that question, we must venture deep under the earth’s surface and understand the process in which diamonds are formed. As mentioned above, diamonds are made up of only Carbon. Carbon is represented as a C on the periodic table of elements. Another substance that is made up of only Carbon is graphite. Graphite is used in pencils as well as lubricants for machines as it is extremely soft. But, one may ask, how does something as soft as graphite on the tip of your pencil have the same chemical composition as the hardest mineral substance on earth, the diamond?This has to do with the way the atoms bond together. Without getting into a major chemistry lesson, it is best to understand that the formation process of a diamond is what influences how tightly the atoms bond together to create the diamond. Deep under the earth’s surface, there are many minerals being formed. There is constant shifting of elements, temperature and pressure. Graphite has strongly bonded atoms, but the layers of each are soft, making it soft and easy to break. The difference between a diamond forming and the formation of graphite is the heat and pressure that is present during their formation.Heat and temperature along with the right combination of chemical elements is how a gemstone is formed. Scientists can find out so much about how diamonds are formed by studying the inclusions of diamonds as well as the different types of rocks that are found around diamond sources. There has been much research performed by scientists to finally confirm that the estimated conditions required for diamond formation are a temperature range of 1652 degrees F to 2372 degrees F (900C - 1300C) and a pressure between 45 kilobars to 60 kilobars. Translation: very high heat, and very high pressure. These conditions are found in very few places in the earth’s mantle which is located between 90 miles and 120 miles (about 140 kilometers and 190 kilometers) below the surface. The best diamond forming conditions exist in areas called cratons, which are the oldest and most geologically stable areas of the earth’s crust. Cratons are located mainly in the center of the continents and have not changed for millions of years and are not located under oceans. The earth’s mantle under the oceans, as far as scientist can understand to this day, does not make up ideal condition for diamond formation. When diamonds are formed, they must be in a location that is rich in carbon. Through many years of research, scientists have determined that diamonds are found in a host rock known as peridotite and eclogite. These two types of igneous rock provide the carbon rich source and are located in an area with the right amount of heat and pressure for diamonds to form. Each host rock releases carbon in different ways and will form different crystal forms and inclusions within the diamonds that are formed. This can be identified with careful scientific inspection.
It is understood that peridotite existed when the earth was formed, and carbon was trapped in this rock from those ancient times. When the rock is heated by the earth, the carbon is released from the host rock. Eclogite is a rock that is closer to the earth’s surface and is not as old as peridotite. The carbon trapped in this rock is assumed to be sourced from organic substances such animals and plants. When Eclogite is brought down further into the earth through subduction, the earth’s inner heat releases the carbon from the host rock. In both cases, when carbon is released into the right conditions of heat and pressure as well as the right chemistry, the carbon atoms will bond together to build diamond crystals. Once a diamond crystal has formed deep in the earth’s mantle, it then needs to be transported up towards the surface. The vehicle that diamonds use to get there is a rock called Kimberlite or Lamprolite. These two rocks are in molten form when melted under the earth’s crust. The earth is constantly shifting and the temperature will cause the molten rock to rise and fall, this is called convection. When the rising molten rock finds a fissure or fracture in the earth’s crust, it will continue to rise and the molten rock begins to speed up. As it is travelling through these areas the rising kimberlite or lamprolite my pick up diamond bearing boulders of eclogite or peridotite and carry them upwards. Most of the time the molten rock is so hot it will dissolve the rock around the diamonds, leaving the diamonds to travel with the rising kimberlite or lamprolite. As the rock continues to move upwards, the pressure becomes lower and lower and the magma speeds up. Once the magma reaches approximately 1.5 miles to the surface, it is estimated that the magma is traveling at about 186 miles/hour (300km/hour). As the magma is travelling, the change in heat and pressure releases gas and other chemical elements such as water vapor and carbon dioxide. The releasing gas is important to the survival of diamonds, as they will protect the travelling crystal from being formed into graphite. The expanding gasses and speed of the travelling magma will drive the mass to blast through the earth’s crust like a cork flying off shaken bottle of champagne. Kimberlite tends to create a carrot shaped tube in the crust and lamprolite causes a more mushroom shaped formation. These are known as ‘pipes’. The pipes do not burst open like a volcano eruption, rather, they will solidify in pipes in the earth’s crust and most of the overlying rock and kimberlite will fall back into the pipe and take the diamond crystals with it resulting in a deep tube like pipe.
Kimberlite pipes can be found mainly in the center of the earth’s continents. There are many kimberlite pipes throughout the world, but only about 1/6th of them are diamond bearing and the ones that are diamond bearing, might not yield enough to be economically profitable to mine and with pipes that have been mined, many of them are not yielding any more diamonds to be economically viable. And to make it even rarer, not all diamond bearing pipes will give gem quality diamonds.