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How are diamonds formed ?

How are diamonds formed ?

It is extremely difficult for even those who are aware of the fact to believe and accept that the chemical composition of a diamond is the same as that of carbon or graphite. This is so because in outward appearance and physical properties diamond is so very different from graphite.

The creation of the priceless natural diamond and the manner in which it reaches places that are accessible to man are processes that can intrigue anyone. Born under extreme conditions of temperature and pressure and that too thousands of miles under the earth’s surface, the diamond is transported violently towards the crust along with other rocks.

Before the diamond can come to you as part of an accessory or a jewelry item, the diamond goes through a process of exploration, discovery and then cutting and polishing so that you can see the real natural shine that it has. This article aims at detailing all the processes and conditions that a diamond goes through before it reaches you in the shape of an engagement ring or an accessory. Everyone in the diamond industry needs to have an appreciation of this process so as to marvel at the wonder of these marvelous gems.

Even though diamonds have been adorning various kinds of jewelry that we wear, history has it that scientists had not unearthed the secret of the formation of a diamond until the late twentieth century. In the last 50 years itself, physicists, gemologists, mineralogists and chemists have been able to put their knowledge together to come up with all that we do know today. The information about how they are formed and transported to the surface has led to a more methodical manner in which diamond sites are being explored and predicted today.

Knowledge about the formation and transportation of these precious stones also helps you appreciate the extreme nature of the components required and the rarity of the condition. It allows you to appreciate the reasons for the high value associated with diamonds. The method and conditions required for creation of this precious stone are very unique and rare and a small missing link will result in a diamond not really being a diamond at all.

To know diamonds are made of carbon and that it is only under specific conditions temperature and pressure is not enough. A comparison can bring to life the extent of extremeness of the situation. Water can freeze and turn into a solid as the temperature reaches below 0 degrees. Water also turns to ice when extremely high pressure is applied to it. If these two parameters working alone can change the physical properties of water, imagine the amount of change that can be initiated when temperature and pressure work together.

Composition – Though it is fairly well known now that diamonds are made from carbon (atomic symbol C) alone, this was not the case earlier. Till the late 1970’s scientists and chemists were baffled about the chemical composition of diamonds. When French chemist Antoine- Laurent Lavoisier had not set the diamond in an oxygen-filled chamber and focused sunlight on it with magnifying glass, we would still have not known what a diamond is made of. The result of Lavoisier’s experiments was astonishing. Instead of finding ash at the base of the jar, which is what he expected, the diamond vanished completely. He could only guess at that moment that the diamond is made of only carbon and that the by product of the experiment was simply carbon dioxide.

An English chemist Smithson Tennant compared the weight of the gas released from a certain weight of charcoal and diamond and found the weight to be same. Even though these results were published in 1797, it took more than two decades for people to be entirely convinced that the chemical composition of a diamond and a piece of charcoal is the same.

To give you an example, graphite is also only made of carbon. However, if you examine their physical properties, they are extremely different. While graphite is used in lead pencils since it soft and dark, a diamond is the hardest substance work. The difference obviously lies in the manner in which the atoms are arranged.  While the carbon atoms in graphite are arranged in strongly bonded layers, a diamond comprises of various three-dimensional structures that bond extremely strongly to give the diamond the hardness that it has.

Temperature and Pressure conditions (how are diamonds formed) – After numerous attempts to find out the exact conditions of temperature and pressure, scientists today know the circumstances under which a diamond gets created. This understanding and technique is used by many to create synthetic diamonds.

For diamond formation, a temperature range of 900oC to 1300oC and a pressure range of 45 kilobars to 60 kilobars is required. A kilobar is a unit used to measure high pressure. To give you an idea 45 kilobars if 45,000 times the atmospheric pressure that one feels normally at seal level.

Once the scientists knew the levels of temperature and pressure that was required to form these gemstones, they studied the various rocks that accompanied diamonds to the surface. Since enough knowledge was existent about the various kinds of rocks and the level that they were found at in the earth’s crust, scientists were able to predict that diamonds were formed somewhere between 90 to 120 miles or 140 kilometers and 190 kilometers below the earth’s surface.

Another aspect that was studied was the different kinds of mineral inclusions in the diamonds to arrive at similar conclusions. It was therefore understood that whenever these conditions occurred in the presence of adequate amounts of carbon, diamond formation was a possibility. The different impurities(minerals) present while formation of diamonds results in fancy colored diamonds like yellow, blue, red etc.

Over time with extended research, it was also understood that if the right temperature and pressure occurred under large geological stable parts of the earth (known as cratons), diamond formation was more likely. Cratons are portions of the earth that exist in the interior parts of the continent. These have been stable and unchanged for more millions of years. Cratons cannot exist under an ocean since the crust is thinner there and the conditions are unstable for diamond formation in these areas.

Source rocks – When a study was made about the various types of rocks that accompany diamonds to the surface, it was found that peridotite and eclogite rocks, two kinds of igneous rocks, accompany diamonds to the surface. These igneous rocks start off in a molten or partially molten state and become hard as they cool while coming closer to the surface of the earth. It is therefore obvious that an understanding of geology is essential for anyone in the diamond industry to start off on a search of likely diamond locations.

The mineral composition, temperature and pressure combinations required for forming them and the amount of carbon supplied by each in the process of formation differs. And therefore the diamonds produced in either case are known to have different crystal structures, formations and inclusions.

Peridotite carbon has been known to be trapped in ancient rocks. It is known that as the rocks melt, they release carbon in the inner layers of the earth. Since there is a large amount of peridotite rock available at those levels and that these are ancient rocks, the process of carbon release is fairly consistent. This means that as we go about our normal daily work, diamonds may be forming under the earth as and when and where the specific temperature and pressure conditions exist.

The carbon released by the ancient eclogite rocks is closer to the surface of the earth. These rocks are younger than the peridotite ones and the carbon mainly comes from organic matter like plants and animals. These decayed plants and animals have probably become part of the eclogite rocks over time and when these rocks move downwards into the earth’s layers this carbon is released by a process called subduction.

Subduction occurs when two large layers of the earths crust are forced into each other. The result of this is that one of the layers is pushed under the other one. As one layer goes below the other one, it gets heated up and the components of the rock melt, releasing carbon in the process. Since the source of this carbon is mainly from the organic matter that has been trapped in the rocks, the release of carbon is not constant.

Irrespective of whether the diamonds are formed due to carbon released by peridotite rocks or eclogite rocks, the fact of the matter remains that both these processes are carried out miles under the surface of the earth. Unless these diamonds are pushed, forced or transported to the earths crust or nearer to the earths crust, there is no way that these diamonds can ever become accessible to man. Sometimes, it takes hundred’s of millions of years for the diamond to make an appearance.

Transportation to the surface – On the earths surface diamonds can be found in kimberlite or lamproite rocks. Though the chemical composition of these two rocks is similar, kimberlite rocks are found in the inner side of cratons while lamproite are found on the edges of the cratons. The process by which these rocks are carried to the surface of the earth is called emplacement. The first diamond emplacement probably took place more than 2.5 billion years ago. The most recent emplacement would probably have occurred about 20 million years ago.

It was thought at some time that these rocks were the ones in which diamonds were produced. And it was not until the time when man discovered the technology to measure a rocks age that it was discovered that the age of these diamond carrying rocks was far lower than the diamonds themselves.

The manner in which the kimberlite and lamproite rocks reach the earth surface is by a process called convection currents. The heat inside the earth keeps the mantle in a molten state. As the magna (molten rock material) heats up, it rises towards the surface of the earth where it is cooled. Cooled magna solidifies and becomes heavy and starts to descend again. As it goes lower inside the earth’s crust, it heats up again. It is only during an upward movement when it meets a fracture in the earth’s crust that continues to move towards the earth’s surface. If such molten magna meets a diamond pouch on the way, it carries the diamonds with it and brings them to the surface of the earth. The diamonds may remain embedded in the peridotite or eclogite rock. But at times the heat of the magna may at times cause the rock to dissolve, leaving the diamonds free of any adjoining rock and these loose diamonds can be retrieved far more easily.

It is important to note that the entire process of the magna gushing out happens fairly quickly. And as the pressure above the emerging molten mass reduces, the speed increases. It is estimated that towards the last 2.5 kilometers the speed of the magna and quickly forming rock is about 300 kilometers per hour. Due to this speed and reducing pressure some of the molten magna expands to gas that consists of mainly water vapor and carbon dioxide. The effect created can be compared to a shaken bottle of soda. As the kimberlite rock forces its way out of the earth’s surface, it forms a carrot shaped tunnel called the pipe. Lamproite emplacements are also similar in nature. However, the pipe formed by them is more like a mushroom shape. These pipes form the specific locations where diamond deposits can be found and mined from.

It is this speed of delivery and the gases that are essential for the survival of the diamonds. It is only due to these that the diamonds retain their atomic bonds and do not convert to graphite, which can happen if there is too much exposure of these diamonds under conditions of high temperature and low pressure.

The destination - As the lamproite or kimberlite rock shoots out, it does so with force. Note that it does not gush out like the volcanic lava in a volcano. It is a mixture of solid rock, gases and ash. A large part of the material that is thrown out due to the speed falls back in the pipe itself and what is left behind is a diamond filled pipe.

Multiple emplacements can occur in the same pipe over years, each set of rough diamonds being different from the older ones.

To get an idea of the preciousness of the diamond and the reasons why it is so valued, one needs to consider the statistics. About 6000 emplacements are known to have happened across the world. Only about a 1000 of these contain diamonds but only about 50 of these pipes are economical viable to mine. Today only about 20 of these mines are being mined.

After having understood the composition, formation, creation, transportation and arrival conditions that are necessary for a diamond to be made available to man, it is only too obvious to understand the reasons for its value. The presence of a diamond close the earth’s surface in itself is a miracle.  And once the rough diamond is retrieved, it goes through a set of other processes that involve cutting, polishing and setting into a piece of jewelry before it actually reaches you. Now that you have understood the circumstances and conditions required you will be better placed to appreciate the beauty and miracle of a diamond.

 

Reference : GIA study material.

 

 

 

 

 

 


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