Diamonds form from coal
Nearly all natural diamonds formed long before coal existed on Earth, in most cases more than a billion years earlier. Diamonds crystallize from inorganic carbon deep in Earth's mantle at depths of roughly 150 to 200 kilometers, under conditions entirely different from the compressed plant material near the surface that eventually becomes coal.
What we know
The claim that diamonds form from coal is a widely repeated piece of geological folklore, appealing partly because both materials are carbon-based and partly because it constructs an intuitively satisfying narrative in which humble, compressed plant matter is dramatically transformed into the most prized of gemstones under sufficient pressure. Geological evidence directly contradicts this appealing story on both timing and mechanism.
Coal forms through the compression and chemical transformation of accumulated plant material, typically from swampy, forested environments, over long periods of geological time, but critically, this process is fundamentally dependent on the prior existence of land plants capable of producing the organic matter that eventually becomes coal. The geological record shows that land plants did not evolve and become sufficiently abundant to produce significant coal-forming plant matter until roughly 350 to 400 million years ago, during the Devonian and Carboniferous periods, the latter era, in fact, taking its very name from the extensive coal deposits formed during that geological period. The overwhelming majority of Earth's coal deposits date from this Carboniferous period and later.
Natural diamonds, by direct contrast, form through an entirely different and far more ancient geological process. They crystallize from carbon-containing material deep within Earth's mantle, typically at depths of roughly 150 to 200 kilometers below the surface, under the extraordinarily high pressure and temperature conditions found at that depth, conditions that cause carbon atoms to bond in the specific tetrahedral crystal lattice structure that gives diamond its characteristic hardness and optical properties. Radiometric dating of mineral inclusions trapped within natural diamond crystals during their formation, a well-established geochronological technique, has determined that the large majority of natural diamonds are extremely ancient, with most falling in an age range of roughly 1 billion to 3.5 billion years old, meaning the overwhelming majority of Earth's natural diamonds had already finished forming well over half a billion years, and in many cases several billion years, before land plants had even evolved, let alone before any coal-forming plant matter existed to be transformed.
This timing mismatch alone is sufficient to rule out coal as a meaningful source material for the vast majority of natural diamonds, since a substance cannot be formed from a source material that did not yet exist. The carbon that does become incorporated into forming diamonds originates instead from other, non-biological sources within the mantle, and geologists studying diamond formation, including detailed carbon isotope analysis of diamond samples, generally attribute this source carbon to inorganic mantle carbon, recycled subducted oceanic crustal material (in some specific diamond formation models), or other deep-Earth carbon reservoirs, entirely distinct from surface plant-derived organic carbon.
Diamonds are ultimately delivered to Earth's surface, where they can be mined, through volcanic eruptions of a specific and relatively rare rock type called kimberlite, which forms narrow, cone-shaped volcanic pipes that transport diamond-bearing mantle material rapidly upward to the surface, a violent geological transport process entirely unrelated to the near-surface sedimentary compression process that produces coal. Geologists and science educators who address the diamonds-from-coal misconception directly generally emphasize this dual point, the radiometric age evidence and the completely distinct formation mechanism and depth, as together providing a thorough and conclusive case against the popular myth, which nonetheless persists in casual conversation largely due to the superficially appealing but ultimately geologically incorrect intuition linking two carbon-based materials through an imagined shared origin story.
Common claims
- Diamonds form from compressed coal.Not supported
- Most natural diamonds are over a billion years old.Accurate
- Coal formation required land plants that evolved much later than most diamonds formed.Accurate
- Diamonds are brought to the surface through kimberlite volcanic eruptions.Accurate