Harmonium was discovered during the Arcane Resurgence. It created such a boom for arcane technologies that it became the most valuable resource in the world. The crystal is thought to be solid remnants of Harmonia's sacrifice during the Fracture, deposited across the Material Plane during the creation of the planes.
Harmonium crystals have many uses that have been discovered. One use is as a substitution for material spell components. Harmonium is mined in its raw form, and in this form, it acts as double its value as a spell component. However, the spell also has a chance of failing and causing a wild magic surge. The mages of the Arcane Resurgence found a way to refine Harmonium into a more stable state. Through magical smithing, they were able to reduce the potency of the crystal in order to make it more stable. It takes more refined Harmonium to substitute as spell components, but it has no chance of failure.
Another use for Harmonium is in magical crafting. The Kainomancers quickly discovered that using Harmonium crystals could greatly increase the speed of crafting magic items. This made the mineral even more valuable.
In the modern day, Harmonium is very rare. Very little is found in the wild anymore, and most is stockpiled by powerful governments. It is thought that Spellhaven has the largest reserve of Harmonium in Chandaria.