Block of Coal

The block of coal is a compact way to store coal, and can be used for decoration and fuel.

As a fuel source, it can smelt 80 items, whereas every coal used one-by-one would only result in 72 items smelted.

Crafting
Blocks of coal can only be crafted with regular coal, not charcoal. It is crafted in the same way as other mineral blocks.

As a crafting ingredient
A block of coal can be crafted back into 9 coal items, making it useful as a storage block.

As a fuel
Blocks of coal can be used as fuel in a furnace. One block of coal smelts 80 items, which is more than the smelting time of the 9 coal items used to craft it. Note that since only 64 items can fit in the furnace at a time, to prevent wasting fuel you should restock the smelting items before they run out, and empty the output slot before it fills up. Hoppers come in handy for automating this, but be warned: using a hopper to unload a furnace forfeits any experience from the smelting.

Trivia

 * Cannot be crafted using charcoal due to logical issues with crafting and destroying.
 * Blocks of coal cannot be used as fuel for the minecart with furnace.
 * A full stack of coal blocks can cook for a little over fifty times as many items as a bucket of lava.