Diamonds are made from coal under pressure

What actually happened

The dating settles it. Most gem diamonds are between 1 and 3.5 billion years old. Land plants appeared roughly 450 million years ago, and coal beds are largely Carboniferous, around 300 million. The great majority of diamonds predate the raw material by billions of years, so the story is chronologically impossible for them.

Coal is also in the wrong place. It sits in sedimentary layers within a few kilometers of the surface, and diamonds form at depths around 150 kilometers, at roughly 900 to 1,300 degrees Celsius and 45 to 60 kilobars. They reach us by riding kimberlite eruptions upward at high speed, fast enough that they do not have time to relax into graphite. The carbon involved is primordial mantle carbon, plus some subducted carbonate.

There is a narrow exception worth being honest about. Some diamonds do carry a carbon isotope signature suggesting recycled organic matter dragged down by subduction, so biological carbon can end up in a diamond by a route that runs through a subduction zone over hundreds of millions of years. That is not "coal squeezed into diamond," and the classroom version, complete with the picture of a lump of coal next to a ring, is a myth the Geological Survey and the GIA have both had to correct repeatedly.