How Does a Lump of Coal Become a Diamond?

A lump of coal turning into a diamond is a popular misunderstanding regarding the nature of natural diamond formation. Coal does not actually turn into a diamond in practice, though both are made from carbon compounds. Diamonds are formed from ancient pressured carbon hundreds of miles below the earth's surface.

• Coal is formed from the carbon of deceased plants several miles below the earth's surface. It is located in flat-laying sediment deposits. As coal requires plant matter to form, coal itself cannot be older than the world's first plants. Diamonds, however, are dated to be much older than any known date for the first plant appearances. Time scale wise, plants and thus coal are dated on a scale of hundreds of millions of years, while all natural diamonds were first formed billions of years ago. This means that every diamond is a small piece of the earth's ancient history.

• Diamond formation requires the high-pressure and extremely hot temperatures allowed by relative proximity to earth's core. The carbon that creates diamonds is pushed hundreds of miles deep beneath the earth's crust into the mantle. Coal is formed at much shallower depths, and does not require the same level of pressure.

• The nature of the organic material that compromises coal makes it full of impurities, while diamonds are made of pure compacted carbon compounds. Carbon in diamonds is thought to be from the initial formation of the earth's surface.

• Despite all of the scientific reasons for why coal does not normally naturally turn into diamonds, it may be possible that coal provides the carbon that eventually becomes diamonds in some rare natural situations. Subduction zones that form between tectonic plates sometimes push rock material deeper into the earth's surface at high pressure and high temperatures. If coal is present in this environment, it could be a carbon source that eventually contributes to diamond formation.