Coal

Coal is a dark, stony substance used for smelting steel. It is also used for gaining Thorium Brotherhood reputation.

Source
The primary source for coal is from either a trade supplies or blacksmithing supplies vendors.

Mining

 * Smelt Steel

As a quest objective

 * Making Amends - Repeatable reputation quest for Everlook, requires 4 coal.
 * Replacement Phial - Uldaman Quest requiring 1 coal.
 * Restoring Fiery Flux Supplies via Kingsblood - Repeatable quest at Thorium Point requiring 1 coal
 * Restoring Fiery Flux Supplies via Heavy Leather - Repeatable quest at Thorium Point requiring 1 coal
 * Restoring Fiery Flux Supplies via Iron - Repeatable quest at Thorium Point requiring 1 coal