Description (from Recipaedia)
A chunk of pure coal. Can be used for fuelling the furnace and as an ingredient of various substances, like gunpowder. Used as a region when reducing metal ores to pure metals. Also used as a pigment for making darker hues of paint. In absence of mineral coal, you can burn wood in an oxygen=poor atmosphere in the furnace and obtain a similar substance: charcoal.
Obtaining
Like diamonds, coal can be gained in several ways:
Mining: Up to 3 chunks may be extracted from one coal ore block with any tool.
Charcoal: Putting 2 wood in furnace input slots will transform the blocks into a coal chunk.
Blocks: Putting a solid coal block into any crafting interface will split it into 9 chunks. Coal blocks can be found naturally in some heavy coal ore veins.
Notes
Can be thrown, like other chunks. Will only do minimal damage.
It is better to use Coal Chunks for fuel instead of Solid Coal Blocks since Solid Coal Blocks only have a fuel value of 600 each. Where 9 Coal Chunks have a total value of 720 fuel value, better than Solid Coal Blocks by a total of 120 more fuel values.