Use:

Coal, as fuel for furnaces, is a very efficient commodity, one coal can be smelted eight units of commodity.

Another important function of coal is to make torches, which are essential for underground exploration. While there are other sources of permanent lighting in the game, none are as accessible and convenient as torches. (A pack of coal and eight pieces of wood can make 256 torches, and you'll find plenty of coal along the way.)

Coal mines can be found in the main world at altitudes less than 132 and are often found on exposed stones. It is the most abundant mineral resource, each lode can dig 1-40 coal mines or more.

Nine coals make one coal block. Coal block has greater smelting efficiency than coal: one coal block can smelt 80 units of articles, equivalent to the smelting efficiency of ten coals. However, the use of coal blocks is only necessary when smelting articles of more than 80 units.