Coal
Coal is a combustible sedimentary rock composed mostly of carbon along with hydrogen, oxygen, and trace minerals, formed from the compaction and chemical alteration of ancient plant material. Unlike true minerals, it lacks a fixed crystal structure or chemical formula, and its properties vary with rank, the degree of transformation from the original plant matter. It ranges from soft, brown lignite to hard, glossy anthracite.
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Coal properties at a glance
| Type | Sedimentary Rocks |
|---|---|
| Mohs hardness | 1–2.5 |
| Crystal system | Amorphous |
| Luster | Dull (lignite) to vitreous (anthracite) |
| Streak | Black to brownish black |
| Fracture | Conchoidal to uneven |
| Specific gravity | 1.1–1.8 |
| Transparency | Opaque |
| Common colors | Black, Brownish black |
| Rarity | common |
Geology & background
Coal forms in swampy, oxygen-poor environments where dead plant material accumulates faster than it can decompose, gradually burying peat under sediment. Heat and pressure over millions of years drive off water and volatile compounds in stages, converting peat first to lignite, then bituminous coal, and finally anthracite as carbon content rises and structure becomes more ordered. Major coal-bearing sequences, including Pennsylvania's anthracite fields and the vast coal basins of Europe, formed from tropical swamp forests hundreds of millions of years old. Rockhounds and collectors often value coal for the plant fossils, such as fern fronds and bark impressions, preserved within it, rather than for the coal itself. Anthracite is prized for its glossy, almost metallic luster and conchoidal fracture, while lower-rank lignite is dull, crumbly, and visibly retains woody texture.
How to identify Coal
Coal ranges from dull brown lignite to shiny black bituminous coal and glossy, hard anthracite, with luster and hardness increasing with rank. Streak is dark brown to black, hardness runs roughly 1 to 2.5, and higher-rank coal breaks with a conchoidal fracture while lower-rank coal is more crumbly and earthy. Plant fossil impressions are common and diagnostic. Coal can be confused with jet, a compact form of fossilized wood, or with black shale, though coal is distinctly lighter and more combustible.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Coal is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Coal worth?
| Entry-level specimen | $1 |
|---|---|
| Typical collector specimen | $10 |
| Premium specimen | $75+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Field identification
- Fossil and depositional environment study
- Construction or aggregate when suitable
- Education
Interesting facts
- Anthracite, the highest coal rank, contains over 90 percent carbon and burns with far less smoke than lignite.
- Most of the world's coal formed from swamp forests during the Carboniferous period, roughly 300 to 360 million years ago.
- Coal seams often preserve detailed fossil impressions of ferns, bark, and leaves from the plants that originally formed them.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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