Coal ball
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A coal ball is a type of concretion, varying in shape from an imperfect sphere to a flat-lying, irregular slab. Coal balls were formed in Carboniferous Period swamps and mires, when peat was prevented from being turned into coal by the high amount of calcite surrounding the peat; the calcite caused it to be turned into stone instead. As such, despite not actually being made of coal, the coal ball owes its name to its similar origins as well as its similar shape with actual coal.
A coal ball
Sir Joseph Dalton Hooker, who along with Edward William Binney was the first to report on coal balls
Calcite (middle) and dolomite (top and bottom) are common materials found in coal balls.
A coal ball from southern Illinois