What is "coal ball"

n. A nodule of plant material permeated with minerals found mostly in bituminous and anthracite coal seams

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.

Coal balls often preserve a remarkable record of the microscopic tissue structure of Carboniferous swamp and mire plants, which would otherwise have been completely destroyed. Their unique preservation of Carboniferous plants makes them valuable to scientists, who cut and peel the coal balls to research the geological past.

In 1855, two English scientists, Joseph Dalton Hooker and Edward William Binney, made the first scientific description of coal balls in England, and the initial research on coal balls was carried out in Europe. North American coal balls were discovered and identified in 1922. Coal balls have since been found in other countries, leading to the discovery of hundreds of species and genera.

Coal balls may be found in coal seams across North America and Eurasia. North American coal balls are more widespread, both stratigraphically and geologically, than those in Europe. The oldest known coal balls date from the Namurian stage of the Carboniferous; they were found in Germany and on the territory of former Czechoslovakia.