Coal

Coal was considered a simple rock for aeons. It had no use in magic rituals and no value as a prized gemstone. That all changed when the first steam engine was constructed by a human engineer named Marth Deiz. He was the first one to burn coal to fuel a machine. Soon, humans realised that coal could fuel many more things, by burning it and producing steam. From there on, coal's monetary value skyrocketed, as human made factories required large amounts of it in order to run.
Nowadays, humans rely on the dwarven colonies of the north in order to get a steady supply of coal. There have been , however, some human mines that tend to use cheap labour and are often criticised about cutting costs to the detriment of the miners. They are considered a dreadful place to work at, and most people there don't tend to live long. Rumour has it that child labour is also a very common theme, but no one would dare utter this, lest they want to incur the wrath of the Coalition.
When coal is burned, it produces a black smog that causes respiratory issues when inhaled for a prolonged time. The black smog is extremely common in human cities that have a lot of factories such as Voyport and the Arkbert Citadel. In the night, the smog is so thick in Voyport, they say it is like the whole city is engulfed in thick otherwordly fog. The inhabitants of those towns, often suffer from maladies, caused by the lack of fresh oxygen. The most known one is called "black lungs" and starts as an innocent cough, but soon develops into small tears on the individual's lungs, causing a slow death that happens overtime. Elves are especially prone to this, so they tend to completely avoid these places, calling them "Dal' Var Hevim", or "Slums of Burnt Coal" in the Common language.