Coal balls are permineralised life forms that are full of calcium, magnesium and occasionally iron sulfide. They generally have a round shape. Coal balls are not made of coal, even though they have the name "coal ball".
In 1855, two English scientists, Joseph Dalton Hooker and Edward William Binney, found coal balls in England. Because of that, the initial research on coal balls was done in Europe. Coal balls were found and identified in North America in 1922. Since then, coal balls have been found in other countries and several theories on their formation have been proposed.
Marie Stopes and D.M.S. Watson also examined coal ball samples. They agreed that coal balls formed in situ. They also added that interaction with a marine environment was necessary for a coal ball to form.
In 1962, Sergius Mamay and Ellis Yochelson found signs of marine animal remains in North American coal balls.
The quality of the preservation of organic material depends on the speed of the burial process and the degree of compression before undergoing permineralisation. Generally, coal balls resulting from remains that have a quick burial with little decay and pressure have a higher preservation degree. However, plant remains in most coal balls show various signs of decay and collapse.
Thin sectioning was an early procedure used to analyse fossilised material contained in coal balls. The ball was cut into thin sections with a diamond saw. Then it was flattened and polished with an abrasive. Then it was examined under a microscope. This is the procedure which was carried out by Hooker and Binney. The time needed, and the poor quality of samples produced, led to a more convenient method.
- Stopes, Marie C.; Watson, D.M.S. 1909. On the present distribution and origin of the calcareous concretions in coal seams, known as 'coal balls'. Philosophical Transactions of the Royal Society 200 (262–273): 167–218. 
- Mamay, Sergius H.; Yochelson, Ellis L. 1962. Occurrence and significance of marine animal remains in American coal balls. Shorter Contributions to General Geology 193–224.
- Seward A.C. 2010. Plant life through the ages: a geological and botanical retrospect. Cambridge University Press. ISBN 1-108-01600-6
- Scott, Andrew C. & Rex G. 1985. The formation and significance of Carboniferous coal balls. Philosophical Transactions of the Royal Society. B 311 (1148): 123–137. 
- 1. Peels are got by cutting the surface of a coal ball with a diamond saw, grinding the cut surface on a glass plate with silicon carbide to a smooth finish. [So far, this first part is the same as the old technique) 2. The cut surface is etched with hydrochloric acid. The acid dissolves the mineral matter from the coal ball, and leaves a projecting layer of plant cells. After applying acetone, a piece of cellulose acetate is placed on the coal ball. After drying, the cellulose acetate can be removed from the coal ball with a razor and the obtained peel can be stained with a low-acidity stain and observed under a microscope. Up to 50 peels can be extracted from 2 millimetres (0.079 in) of coal ball with this method.