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A coal ball is a type of concretion that is found in coal and consists of plant debris (peat), which has been permineralised by calcite. Coal balls vary in shape from imperfectly spherical to flat-lying, irregular slabs. These concretions formed by the early permineralisation of peat by calcite in Carboniferous Period swamps and mires prior to its alteration to coal. They derive their name from their association with coal and their often imperfectly spherical shape. Because they formed in prehistoric peats prior to them becomingcoalified, they often preserve remarkable record of the tissue structure of Carboniferous swamp and mire plants. Paleontologists have to cut and cut and peel open the coal balls to extract the fossils inside.

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 later discovered and identified in 1922. Since then, coal balls have been found in other countries, and they have led 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 relatively widespread, both stratigraphically and geologically, as compared to coal balls from Europe. The oldest known coal balls date from the Namurian stage of the Carboniferous, and they were found in Germany and on the territory of former Czechoslovakia. (see more...)