Is "Diamonds are formed from compressed coal" true?
Science class loved the tidy story: squeeze coal hard enough and you get a diamond. Both are carbon, so the leap felt obvious. Most natural diamonds crystallized far deeper and far earlier than coal ever existed. Mantle carbon under extreme heat and pressure, later blasted up in rare volcanic pipes, is the real path for nearly every gem ever mined.
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What you were taught
Diamonds are formed from compressed coal
What we know now
Diamonds do not form from coal. Nearly all commercial diamonds crystallized in Earth’s mantle about 150 kilometers or more underground, at temperatures of at least about 1050°C, then rose in rare kimberlite or lamproite eruptions. Most dated diamonds are Precambrian—older than the earliest land plants (about 450 million years ago), which are the source of coal. Coal is a near-surface sedimentary rock rarely buried more than a few kilometers, so it is not a plausible carbon source for mantle diamonds.
Was "Diamonds are formed from compressed coal" taught in school?
Yes — and not as a joke question on a quiz. This science claim showed up in textbooks, worksheets, and classroom posters through the 1990s, which is why so many people still remember it as settled fact long after the science moved on.
Is "Diamonds are formed from compressed coal" true?
No. Diamonds do not form from coal. Nearly all commercial diamonds crystallized in Earth’s mantle about 150 kilometers or more underground, at temperatures of at least about 1050°C, then rose in rare kimberlite or lamproite eruptions. Most dated diamonds are Precambrian—older than the earliest land plants (about 450 million years ago), which are the source of coal. Coal is a near-surface sedimentary rock rarely buried more than a few kilometers, so it is not a plausible carbon source for mantle diamonds. If you want the primary citation, start with Geology.com - How Do Diamonds Form? They Don't Form From Coal!
When was this understanding updated?
The evidence had largely shifted by around 1998. Schools don't flip overnight, though — plenty of classrooms kept teaching the older version for years after researchers had already moved on.