Sedimentary rock

Coal (most likely Bituminous or Anthracite)

Coal (primarily composed of carbon with varying amounts of other elements)

Hardness: 0.5-2.5 (very soft to soft), Color: Black or dark brown, Luster: Dull, submetallic, or vitreous (especially anthracite), Crystal structure: Amorphous (lacks true crystal structure), Cleavage: None (fractures irregularly or conchoidally in anthracites), Specific Gravity: 1.1-1.5 (varies with type of coal)

Hardness: 0

Identified October 24, 2025More sedimentary rock →

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Physical properties

Formation & geological history

Formed from peat through a process called coalification. Plant material accumulates in swamps or bogs, is buried by sediment, and subjected to heat and pressure over millions of years. This process removes water, oxygen, and hydrogen, increasing the carbon content. The geological age ranges from the Carboniferous and Permian periods (for most major deposits) to younger Cenozoic deposits.

Uses & applications

Primarily used as a fossil fuel for electricity generation, industrial processes (e.g., steel production), and historically for heating. Less commonly used as a carving material for ornamental objects (e.g., jet, a form of lignite).

Geological facts

Coal is the largest source of fuel for the generation of electricity worldwide. The rank of coal (lignite, sub-bituminous, bituminous, anthracite) depends on its carbon content and the degree of coalification it has undergone. Anthracite is the highest rank of coal, with the highest carbon content and energy density, and a shiny, almost metallic luster.

Field identification & locations

Identify it in the field by its black to dark brown color, relatively soft nature (it can often be scratched with a fingernail or knife), and its characteristic dull to submetallic luster (though anthracite can be shiny). It often has a layered or blocky appearance. It is commonly found in sedimentary basins worldwide, particularly in regions with ancient swamp environments. Tips for collectors: It is relatively fragile and can be dusty. Anthracite is harder and cleaner to handle than lower-grade coals.

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