Lignite Identification Guide

What Lignite Looks Like

Lignite, or "brown coal," is the lowest rank of true coal, sitting between peat and sub-bituminous coal in the coalification series. It is a soft, brownish-black sedimentary rock formed from compacted, partially altered plant matter. Lignite often retains visible woody texture — you can sometimes still see plant fibers, bark, or tree-ring structure.

• Color: brown to brownish-black, duller than higher-rank coals
• Luster: dull to slightly waxy; not glassy or bright
• Streak: brown (a key separator from black bituminous coal)
• Texture: often shows woody or layered (fibrous) structure; may have earthy feel
• Weight: light for its size; high moisture content

Step-by-Step Field-ID Checklist

1. Check the color and streak. Scrape it on unglazed porcelain — lignite leaves a brown streak, while bituminous and anthracite coals leave black streaks.
2. Look for woody texture. Visible plant fibers, bark, or wood grain strongly suggest lignite (often called "xyloid lignite" when very woody).
3. Heft it. Lignite is light and feels relatively soft and fragile; it crumbles more easily than hard coal.
4. Test for drying cracks. Fresh lignite holds a lot of water and tends to crack, shrink, and disintegrate as it dries out.
5. Try to scratch/crumble it. It is soft enough to mark with a fingernail or break apart by hand.

Key Diagnostic Tests

• Mohs hardness: very low, roughly 1–2.5; easily scratched.
• Streak: brown to brownish-black — diagnostic versus harder coals.
• Specific gravity: low, around 0.6–1.3; noticeably light.
• Fracture: earthy, crumbly; lacks the bright conchoidal fracture of anthracite.
• Burn test (where safe): ignites readily with a smoky flame and earthy/woody smell, more than higher-rank coals.
• Non-magnetic; no acid reaction.

Common Look-Alikes and How to Tell Them Apart

• Peat: even softer, browner, fibrous and spongy, with obvious undecomposed plant matter and very low density; peat is the precursor and is not yet rock-hard.
• Bituminous coal: harder, blacker, brighter luster, black streak, and breaks into blocky pieces; higher energy content.
• Anthracite: hardest coal, sub-metallic glassy luster, black streak, conchoidal fracture — clearly distinct from dull brown lignite.
• Black shale / carbonaceous shale: harder, splits into thin flat layers, and is heavier; gives a gritty rather than woody texture.
• Jet: a compact, polishable variety of fossil wood/lignite that is tougher and takes a high shine, unlike crumbly common lignite.

Where It Is Typically Found

Lignite forms in former swamps and peat bogs that were buried but never deeply enough or heated enough to mature into higher-rank coal. It occurs in extensive, relatively young (often Cenozoic) sedimentary basins. Major deposits are found in the northern Great Plains of the United States (notably North Dakota), Germany, Australia (Victoria's Latrobe Valley), Greece, and many other regions, typically in flat-lying seams within soft sediments.