A common myth is that diamonds are formed from coal. Even though people have tried to transform coal into a diamonds with varying results, this is not how diamonds were formed in nature. Why?

Timeline Does Not Match

Geologists believe that diamonds were formed deep within the earth's mantle 1-3 billion years ago. Coal is an organic fossil fuel that was created from the remains of plant life that formed 290 to 360 million years ago.

Coal was simply not around when diamonds were formed. 

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Composition is Not the Same

Both diamonds and coal are made from carbon, but with stark differences in purities. Diamonds are made of pure carbon, which is unique considering other gemstones have more complex chemical compositions. Only colored diamonds have slight impurities or defects that cause the color, but these impurities are very slight when looking at the overall structure. Coal, on the other hand, is very impure. These impurities make it very difficult to synthesize coal into a diamond, and also make it very unlikely coal ever formed into a diamond naturally. 

Differing Depth of Formation

Most commonly, diamonds were formed deep within the earth's mantle about 90 to 100 miles below ground. Carbon that deep within the earth crystalizes when it experienced extreme pressure and heat from magma. Rare deep source eruptions that happened many years ago quickly forced the diamonds toward the earth's surface.

They were then trapped in the cooling magma that formed into an igneous rock known as kimberlite. Coal is mined at depths of only a few thousand feet at most. 

So despite the folklore of turning a piece of coal into a diamond, it is very unlikely coal ever had any significant impact on the formation of diamonds.

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