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 72 items, the same as 9 coal which went into its construction. 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).

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 72 items, which is equal to the smelting time of the 9 coal items used to craft it.

Trivia

 * Cannot be crafted using charcoal due to logical issues with crafting and destroying in the code.
 * Uses the same texture as block of redstone, but black instead of red.
 * The Block of Coal cannot be used as fuel for the Minecart with Furnace
 * A full stack can cook almost fifty times as many items as a bucket of lava.
 * Was first in the 2.0 April Fools update