How to identify Coal

To correctly identify Coal (Coal), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Typically black to brownish-black. Lignite can be dark brown, while anthracite is jet black.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, waxy, or vitreous (glassy) depending on the rank. Anthracite often has a bright, submetallic to vitreous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Can be earthy, granular, or compact. Often exhibits banding or layering from original plant material. Anthracite is very hard and dense.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous; does not form crystals. It is a rock, not a mineral.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, but often exhibits fracture along bedding planes or jointing. Anthracite can show conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Forms in ancient swamps, peat bogs, and deltaic environments where rapid burial of organic matter occurred under anoxic conditions, preventing complete decomposition. Found in sedimentary basins.

Key Facts

Hardness

1-2.5 (Mohs scale) for lignite to bituminous; 2.5-3 for anthracite. It is relatively soft.

Specific Gravity

1.1-1.8 (varies with rank and ash content). Lignite is typically lower, anthracite higher.

Crystal System

Amorphous (non-crystalline)

Color

Black, dark brown, brownish-black

Luster

Dull, earthy, waxy, submetallic, vitreous

Transparency

Opaque

Fracture

Irregular, conchoidal (especially anthracite), or splintery

Cleavage

None

Composition

Primarily carbon (C), with varying amounts of hydrogen (H), oxygen (O), nitrogen (N), and sulfur (S), plus inorganic ash constituents.

Physical characteristics

• Crystal Habit: Massive, layered, or banded; amorphous
• Cleavage Type: None
• Fracture Type: Irregular, conchoidal, or splintery
• Tenacity: Brittle
• Luster Type: Dull, earthy, waxy, submetallic, vitreous

Explore Coal