Coal is a Vanilla item harvested directly from coal ore blocks. They are the fourth most efficient fuel available for furnaces (after lava buckets, blaze rods, and coal cokes), 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 that is needed for a lot of power generation. Coal can also be macerated to form Coal Dust which can be used in many different recipies.
In Tekkit, a Diamond is equivelent to 64 Coal, and there are many ways to convert them.