Diamonds form from coal
Nearly all natural diamonds predate coal by over a billion years. Diamonds form from inorganic carbon in the mantle at depths of 150 to 200 km, not from compressed plant debris near the surface.
What we know
The popular image of coal being squeezed into diamonds was popularized by Superman comics and films, but it conflicts with basic geology. Most diamonds were formed in Earth's mantle between 1 billion and 3.5 billion years ago, carried to the surface by rare kimberlite and lamproite volcanic eruptions. The earliest land plants, which are the source material for coal, did not appear on Earth until approximately 450 million years ago. Coal itself forms much later than that, primarily from Carboniferous Period plant matter deposited between 300 and 360 million years ago.
Diamond formation requires temperatures of roughly 900 to 1300 degrees Celsius and pressures of 45 to 60 kilobars, conditions found about 150 to 200 km below Earth's surface in the lithospheric mantle. The carbon source for most diamonds is inorganic - dissolved in mantle fluids or present in ancient subducted oceanic crust. Some rare diamonds from extreme depths of 700 to 800 km may incorporate carbon from subducted marine sediments, but even these formed long before coal existed.
There is one narrow exception: tiny synthetic industrial diamonds and some experimental stones are made by subjecting carbon-rich materials (which could in principle include coal) to high pressure. But this has no bearing on natural diamond formation. The fundamental point remains: coal did not make natural diamonds and cannot, because coal came too late in Earth's history.
Common claims
- Diamonds are made from compressed coalFalse - diamonds predate coal by over a billion years
- Both diamonds and coal are made of carbonTrue - but different carbon sources, conditions, and ages
- High pressure on coal can produce diamondsFalse in nature - industrial synthesis uses pure carbon, not coal