Identification guide · Sedimentary Rocks
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.
Coal identification checklist
| Color | Black, Brownish black | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Dull (lignite) to vitreous (anthracite) | Fresh surfaces shine like glass; a dull surface usually means weathering, so chip or wet a corner. |
| Transparency | Opaque | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 1–2.5 | A fingernail (Mohs 2.5) scratches it easily. |
| Streak | Black to brownish black | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Fracture | Conchoidal to uneven | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 1.1–1.8 | Noticeably light in the hand for its size (specific gravity about 1.4500000000000002). |
| Crystal system | Amorphous |
Extra tests for Coal
Glass check. Coal shares a glassy luster and conchoidal fracture with man-made glass. Glass sits near Mohs 5.5 and often contains round gas bubbles, so a hardness test and a hand lens separate them.
Coal look-alikes and how to tell them apart
Jet is harder (Mohs 3) than Coal (Mohs 1–2.5), so Jet will scratch Coal and not the other way round. A fingernail (Mohs 2.5) scratches it easily. Coal vs Jet →
Both sit near Mohs 1–2.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Coal vs Shale →
Both sit near Mohs 1–2.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Coal vs Anthracite →
Quartz is harder (Mohs 7) than Coal (Mohs 1–2.5), so Quartz will scratch Coal and not the other way round. A fingernail (Mohs 2.5) scratches it easily.
Coal identification FAQ
How can you tell if a rock is Coal?
Check the color (black and brownish black), the dull (lignite) to vitreous (anthracite) luster, a Mohs hardness of 1–2.5 and a black to brownish black streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Coal?
Coal has a Mohs hardness of 1–2.5. A fingernail (Mohs 2.5) scratches it easily.
What is the difference between Coal and Jet?
Coal (Mohs 1–2.5, dull (lignite) to vitreous (anthracite) luster) and Jet (Mohs 3, waxy luster) look alike, but hardness, streak and heft usually separate them. See the full Coal vs Jet comparison for a side-by-side table.
Is Coal worth anything?
Entry-level Coal specimens sell for around $1, typical collector pieces about $10, and premium examples $75 or more depending on size, quality and locality.
What color is Coal?
Coal is most often black and brownish black. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
