Coal is a Vanilla item harvested directly from coal ore blocks. It is the fourth most efficient fuel available for Furnaces (after lava buckets, blaze rods, Coal Coke and the Fuel Can), being able to smelt eight units of ore per lump of coal. Coal lumps and charcoal are also the only fuels accepted by powered minecarts (providing approximately four minutes of transit each).
Coal ore can be found at any elevation, wherever there is Stone. It is the most abundant resource and can be found in veins of 5 or more (sometimes up to 70).
In Tekkit, it is also found in the nether and is one of the ores needed for a lot of power generation. Coal can also be macerated to form Coal Dust which can be used in many different recipes.
In Tekkit, a Diamond is equivalent to 64 Coal, and there are many ways to convert them.