Coal Identification Guide
A field guide to identifying coal — combustible sedimentary carbon rock — by its low weight, black streak, and how to separate it from shale and obsidian.
Read the full Coal encyclopedia entry →
What Coal Looks Like
Coal is a black to brownish-black combustible sedimentary rock formed from compressed, altered plant matter. Its appearance varies with rank: lignite (brown coal) is dull brown-black and often shows woody texture; bituminous coal is black with alternating dull and bright (glassy) bands; anthracite is hard, jet-black, and bright with a near-metallic, glassy luster. Coal is notably lightweight for its size and often leaves sooty marks on your hands.
Step-by-Step Field ID Checklist
- Check color and luster. Black to brown-black; dull (lignite) to bright banded (bituminous) to glassy (anthracite).
- Heft it. Coal feels light — lower density than most rocks.
- Test the streak. Rub on unglazed porcelain: coal gives a black to brown-black streak.
- Check for marking. Softer coal blackens your fingers; anthracite is cleaner.
- Look for layering. Many coals show fine horizontal banding (vitrain/durain layers) or plant fossils.
- Test hardness. Soft (lignite/bituminous ~1–2.5); anthracite is harder (~2.5–4) and shinier.
Key Diagnostic Tests
- Hardness: 1–2.5 for most coal; anthracite up to ~4.
- Streak: Black to brown-black — a key separator from black shale (gray streak).
- Density: Low, ~1.1–1.5 g/cm³; it feels light and may even be close to floating in lignite.
- Fracture: Bituminous coal often shows blocky cleat fractures; anthracite breaks conchoidally.
- Combustibility: It burns — the definitive property (test responsibly; smell of burning).
- No acid reaction, non-magnetic.
Common Look-Alikes and How to Tell Them Apart
- Black shale: Shale is denser, gives a gray to grayish-brown streak, splits into thin flat plates, and does not burn readily. Coal is lighter with a black streak.
- Obsidian: Volcanic glass is hard (5–5.5), gives a white streak, and has bright conchoidal fracture; coal is soft with a black streak.
- Jet: Jet is essentially a hard, compact form of fossil wood (a coal-like material); it is tougher, takes a polish, and is used as a gemstone, but is closely related.
- Anthracite vs bituminous: Anthracite is harder, brighter, breaks conchoidally, and burns cleaner; bituminous is softer with dull/bright bands.
- Manganese oxide / hematite: These are much denser and heavier; hematite gives a red-brown streak.
Where It Is Found
Coal forms from ancient swamp and peat deposits buried and compressed in sedimentary basins. Major occurrences include the Appalachian and Illinois basins (USA), the Powder River Basin (Wyoming), the UK, Germany, China, Australia, and Russia. Look in sedimentary sequences with sandstone and shale, in seams or float along outcrops and streambanks.
Frequently asked questions
How can you tell if a rock is coal?
Coal is black to brown-black, lightweight, leaves a black or brown-black streak, is soft (it can mark your hands), often shows banding, and is combustible — it burns, unlike look-alike rocks.
Coal vs black shale — what's the difference?
Coal is lighter, gives a black streak, and burns. Black shale is denser, splits into flat plates, gives a gray streak, and does not burn easily.
What are the types of coal?
By increasing rank: lignite (dull brown), sub-bituminous, bituminous (banded dull/bright black), and anthracite (hard, glossy, near-metallic black). Higher rank means harder and shinier.
Is coal the same as obsidian?
No. Obsidian is hard volcanic glass with a white streak and conchoidal fracture; coal is a soft, light sedimentary carbon rock with a black streak that burns.
Coal identified by the community
Recent Coal specimens identified with Rock Identifier.
