01 / Origin story
How does coal form?
Coal is an organic sedimentary rock formed from ancient plant material — typically swamp vegetation — that was buried before it could fully decompose, then compressed over millions of years, progressively driving out water and concentrating carbon.
02 / Read the clues
How can you identify coal?
Use several clues together.
One photograph shows one specimen, not every variation.
- 01Black to dark brown
- 02Lightweight relative to its size, compared to most rock
- 03Dull to slightly shiny depending on type (lower grades like lignite are duller; higher grades like anthracite are shinier and harder)
- 04Burns — obviously not something to test casually, but it's the defining trait
03 / Commonly confused with
A distinctive combination.
Not easily confused with other common rocks once you know what to look for — the combination of color, light weight, and organic origin is fairly distinctive.
04 / Out in the world
Where it’s found.
Ancient swamp and wetland deposits, now major coal-producing regions — Appalachia, the Powder River Basin in Wyoming, and similar basins worldwide.
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