Coal is a vanilla item. While it's availabe on layers 1-128, it will be mostly found in layers 5-52. There are about 143 coal ore blocks per chunk (in vanilla minecraft). Coal comprises around 1% of all stone ore. regardless of the altitude. Coal veins can vary greatly in size.
The Coal Ore itself can be extracted using the Silk Touch enchantment on a pickaxe, by using a Rock Cutter or other Silk Touch methods. The ore can then be processed in a Macerator to receive 2 coal dust instead of the normal 1 coal dust when mining the coal directly yielding twice the normal amount of coal dust gained, or an Industrial Grinder, giving a tiny pile of thorium dust as well as two coal dust.
Coal is a Vanilla item harvested directly from coal ore blocks. They are the fifth most
efficient fuel available for Furnace (after lava buckets, blaze rods, coal cokes and IndustrialCraft Fuels), 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. When in a furnace, coal can smelt up to 8 items.
IMPORTANT: coal is a material needed for the very powerful Nano Suit, which needs well over 100 pieces for a complete set. Not even charcoal can be used for that suit, so always try to make your torches with charcoal instead. Do not you coal (OR charcoal for that matter) as fuel in furnaces or generators, since scaffold is an item that is really easy to make, and has proved to be twice as efficient as a fuel than coal or charcoal!