Diamonds are not squeezed coal, and most of them are older than plants

Everything here earned its place by changing an outcome. Nothing about how natural diamonds form is included to round the number up.

The tidy classroom version

The story runs that coal, given enough time and pressure, is compressed into diamond, and both are indeed made of carbon. It is a memorable pairing, contrasting the cheapest and most valuable forms of the same element in a single sentence.

The image was reinforced for generations by comic books and films in which a sufficiently strong character crushes coal into a gem. The chemistry is not wrong, since both really are carbon arranged differently, but the geology behind the transformation is invented. It is a good example of a simplification that is not a lie about any single fact yet produces a completely false picture.

Where diamonds actually form

Natural diamonds crystallise well over a hundred kilometres down, in the mantle beneath the oldest and thickest parts of continents. The pressures and temperatures required are found nowhere near the shallow sedimentary layers where coal seams sit. They reach the surface only because occasional violent volcanic eruptions carry mantle material upwards fast enough that the crystals survive the journey.

Trace it back and the rock those eruptions leave behind is what miners look for, which is why diamond mines are pipes rather than seams. Nothing about that process involves burying a coal bed and waiting, because a coal bed pushed that deep would be destroyed rather than converted.

The timing problem

Coal is compacted plant material, so it cannot exist before land plants do, and land plants are a comparatively recent development. Most natural diamonds are far older than that, with typical ages running to a substantial fraction of the planet's history.

Where the story actually begins, a diamond that predates the first forest by a billion years cannot have been made from a forest, whatever the pressure. This is the cleanest disproof available, because it does not require any argument about mechanism at all. Dating comes from tiny mineral inclusions trapped inside the stones, which is one reason flawed diamonds are scientifically more valuable than perfect ones.

What the carbon actually was

Some diamonds carry a chemical signature suggesting their carbon came from the mantle itself, present since the planet formed. Others carry signatures pointing to carbon that was once at the surface and was dragged down again where one tectonic plate slid beneath another. That surface carbon can include marine carbonate and organic material from ancient seafloor sediment, so a biological origin is sometimes genuinely involved.

The path is long and indirect, running through subduction and mantle residence over hundreds of millions of years rather than through a coal seam. It is a more interesting story than the myth, since it means some diamonds contain carbon that has completed a full circuit of the planet.

What pressure alone will do

Synthetic diamonds are made industrially, and the process does use high pressure and temperature, which is where the myth finds some support. The starting material is normally graphite with a metal catalyst, not coal, because purity matters enormously and coal is full of other elements.

A second industrial route grows diamond from carbon-containing gas at low pressure, which contradicts the idea that pressure is the essential ingredient. Both methods produce material that is chemically and physically diamond, distinguishable from mined stones mainly by growth patterns and trace impurities. So the pressure part of the myth is roughly right for factories and roughly wrong for the planet.

Printing a correction has a poor record of taking a myth out of circulation.

Why the myth is useful anyway

It has become a metaphor rather than a claim, standing for the idea that pressure improves people, which is why it survives correction. Nobody using it that way is making a geological assertion, and pointing out the science can be beside the point. The trouble comes when it appears in teaching materials, where it displaces a genuinely spectacular account of mantle chemistry and volcanic delivery.

The real version has more of everything the metaphor wants, including enormous timescales, extreme conditions and a violent ascent. Trading a false simplification for a true story that is more dramatic is the easiest exchange in science teaching.

Everything above, in order of what to do first

1. The tidy classroom version. The story runs that coal, given enough time and pressure, is compressed into diamond, and both are indeed made of carbon.
2. Where diamonds actually form. Natural diamonds crystallise well over a hundred kilometres down, in the mantle beneath the oldest and thickest parts of continents.
3. The timing problem. Coal is compacted plant material, so it cannot exist before land plants do, and land plants are a comparatively recent development.
4. What the carbon actually was. Some diamonds carry a chemical signature suggesting their carbon came from the mantle itself, present since the planet formed.
5. What pressure alone will do. Synthetic diamonds are made industrially, and the process does use high pressure and temperature, which is where the myth finds some support.
6. Why the myth is useful anyway. It has become a metaphor rather than a claim, standing for the idea that pressure improves people, which is why it survives correction.

The takeaway

The carbon was already ancient before the first tree existed, which makes the coal story not just wrong but far too small.

The satisfying version of a story is the one that travels, which is the whole problem.