sedimentary

Coal

Coal (Sedimentary Rock / Organic Rock)

Hardness: 1-2.5 on Mohs scale. Color: Black to brownish-black. Luster: Dull to sub-metallic or vitreous. Crystal structure: Amorphous, non-crystalline organic rock. Cleavage: None, usually exhibits conchoidal or uneven fracture. Specific gravity: 1.1-1.5.

Hardness: 1-2
Color: Black to brownish-black
Luster: Dull to sub-metallic or vitreous

Identified July 29, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from compressed plant debris accumulated in ancient swamps and wetlands over millions of years, primarily during the Carboniferous period, through the process of coalification (peat to lignite, bituminous, and anthracite coal).

Uses & applications

Primary uses include electricity generation, steel production, and industrial manufacturing. Bituminous coal and anthracite are also sometimes polished and used for low-cost ornamental items or jewelry (jet).

Geological facts

Coal is often called 'buried sunshine' because the energy stored within it was captured by plants from the sun hundreds of millions of years ago. It represents one of the world's most abundant fossil fuels.

Field identification & locations

Easily identified by its dark black color, light weight, soil or dark streak on unglazed porcelain, and relatively low hardness. Commonly found in sedimentary basins worldwide.