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How diamonds are formed
It is difficult to believe that any relation exists between the dark brownish-black lumps of coal and shimmering diamonds. The common element between the two is that they both are made up of carbon. The element carbon exists in the earth in different forms like coal, graphite, diamonds, etc. So the common factor between coal and diamonds is that they contain carbon. Then why they look different. Is it possible that diamonds are formed from coal? Let us find out.
Relation between Coal, Graphite, and Diamonds
Did you know that carbon is one of the most abundant elements found on the Earth's crust? It is found in different allotropic (structurally different) forms that can be broadly categorized as amorphous and crystalline. The most common amorphous forms of carbon are coal, coke and charcoal, whereas graphite and diamonds are the well-known crystalline forms of this element. The amorphous forms lack the crystalline structure that is found in graphite or diamonds.
If coal, diamonds and graphite are made up of carbon, then why do they look different? The answer is that the arrangement of carbon atoms in graphite and diamonds differ, giving them an entirely different look. While carbon atoms are arranged in a hexagonal manner in graphite, in diamonds, each carbon atom is linked to four others in a tetrahedral manner. Hence graphite is like sheets of carbon atoms that are stacked one over the other; carbon atoms in diamonds form a pyramid-like structure.
The flat sheets of the graphite can absorb lights of different wavelengths and hence are opaque and black. The pyramid-like structure found in diamonds lack this ability and is transparent and sparkling.
Coal, which is an amorphous form of carbon, is not made of pure carbon. Apart from carbon, it contains complicated organic molecules of decomposed plant and animal material, which have been compressed for millions of years, in the Earth's crust. Coal is subjected to various geological processes that change its chemical composition to form graphite. This process requires suitable conditions and millions of years to take place.
Diamonds are formed deep inside the earth, where the temperature and pressure are very high. They are transported to the surface of the earth by the molten magma.
Does Coal Become Diamonds?
It is a common fact that natural diamonds are mined from the Earth. Nowadays, we have synthetic diamonds that are made through artificial methods. But there are certain limitations for making diamonds through artificial methods and so, they form only a very small fraction of the total diamond supply.

As far as formation of natural diamonds is concerned, it is a common belief that diamonds are formed from coal. But how far is it true? It is said that a major chunk of natural diamonds are not formed from coal, though, you may find some indirect links between them. In order to understand these links, let us take a look at the natural formation of diamonds.
How Diamonds are Formed Naturally
» We all know that natural diamonds are mined from the Earth. These gemstones are mainly formed in certain parts of the Earth's mantle, at a depth of around 100 to 200 kilometers from the surface.
» In the Earth's mantle, carbon (could be in the form of pure granite) is exposed to very high temperature and pressure, for a very long time, so as to form diamonds.
» It takes millions and millions of years for diamonds to form in the Earth's mantle. For this to happen, the temperature must be around 2000 degree Fahrenheit and the pressure must be 45 and 60 kilobars. Such favorable conditions exist only in some limited regions of the Earth's mantle. Variations in temperature and pressure may affect the formation of diamonds. A dip in the pressure or a rise in the temperature may result in dissolution of the diamonds or partially formed ones.
» The diamonds that lie in the Earth's mantle, come out to the surface, through volcanic eruptions that are deep in origin. The molten magma, along with diamonds and rocks from the mantle, comes out and cools off to form kimberlites that are the major sources of these gemstones.
» It is also possible that the diamonds are transported from the kimberlite sites to distant areas. This happens when the kimberlites erode with time and the diamonds in them are carried away with the resultant sediments to distant places.
» Nowadays, rapid and violent volcanic eruptions do not happen. It is believed that the eruptions that brought out the diamonds must have occurred long time ago, when the Earth was much more hot than today.
» Sometimes, diamonds can be formed on the surface of the Earth, at sites that have sustained meteorite strikes. This is due to the high temperature and pressure produced on the Earth's surface, as a result of collision with large meteorites.
» It has also been noted that some of the meteorites carry very small diamonds that are also found from impact sites. It is inferred that such diamonds are formed by the high pressure and temperature caused by collisions that happen in space. So some sort of carbon source is present in meteorites too.
» A possibility of diamond formation is in the subduction zones, that are formed when one tectonic plate slide beneath another and move down to the Earth's mantle. Such subduction results in movement of materials with carbon, to the Earth's mantle (as far as 150 km from the surface) and form diamonds. According to a recent study, a fluid mixture of carbon, oxygen and hydrogen travels to the mantle and form microdiamonds.
Sources of Carbon for Diamond Formation
In short, diamonds are formed of carbon and so, the basic requirement is a source of carbon. In case of natural diamonds, there are various theories with regard to carbon sources.
» Diamonds may be formed from the inorganic carbon that is originally found in the Earth's mantle. It is believed that such carbon must have been there right from the time of the planet's formation.

» Another source for diamond formation is the materials with organic carbon (from the Earth's surface) that move into the Earth's mantle, due to subduction.
» In case of meteorite impact sites, carbon in the materials on the sites forms the source for diamond formation.

» As far as diamonds on meteorites are concerned, it is difficult to decipher the source of carbon.
Why Diamonds are Not From Coal
Now it is clear that it is carbon that is the main raw material for formation of diamonds. Apart from the inorganic carbon found in the Earth's crust, one of the sources of organic carbon could be coal. This may happen in case of subduction, wherein carbon from the Earth's crust travel to the mantle. Even in case of diamonds formed in the impact sites, coal could be a source of carbon. So you cannot rule out the possibility of the role of coal in formation of diamonds. However, there are various theories that negate this belief. Here are some of them.
» A major chunk of diamonds are formed in the Earth's mantle and they are brought out to the surface through violent volcanic eruptions. The magma travels through volcanic pipes to the surface of the Earth. So these narrow volcanic pipes, the cooled down magma and the igneous rocks form the primary source of diamonds. On the other hand, coal is usually found in sedimentary rocks that are horizontal in nature.
» A theory suggests that most of the diamonds are millions and millions of years old and they are believed to be formed much before the earliest plants on the Earth. Since coal is mainly formed from plant debris, this theory proves that most of the diamonds are not formed from coal.
» Even the role of coal in diamond formation due to subduction of tectonic plates is negated with the theory that subduction mainly happens in oceanic plates, as compared to continental ones. It is contended that in case of oceanic plates, the main sources of carbon are limestone, dolomite and marble. Plant debris is found in meager amounts only.
So these are some of the theories that rubbish the claim that diamonds are formed from coal. However, there are chances that diamonds form from coal, in case of subduction. Conversion of coal to diamonds is a natural process that takes millions of years. Even though there are carbon atoms in coal, the amount of impurities are so high, that it takes millions and millions of years for the coal to transform to its nearly purest form - graphite. The conversion of graphite to diamonds takes some more millions of years. Presence of coal in impact sites may also result in formation of diamonds. Even if you want to make diamonds from coal through artificial methods, the resultant diamond will be of very poor quality, due to the high level of impurities.

In short, chances of forming diamonds from coal, are very remote. So it is believed that most of the diamonds are not formed from coal.