Aetherium is a rare, blue luminescent crystal found in Dwemer city ruins. It was originally discovered by the Dwemer when they were mining in Blackreach, but after enough was collected, it began to spread across Skyrim in multiple Dwemer cities. It possesses natural magical qualities, though it is almost impossible to work with.
The items produced from Aetherium are said to be extremely powerful artifacts imbued with very powerful enchantments.
Very little is known about the properties of Aetherium other than its luminescent blue hue and its natural, strong magical aura. Alchemically, it is inert and research suggests it was required to be stored outdoors as it was harmonically volatile. 
Dwemer studyEditSometime after 1E 221, the Dwemer of Skyrim discovered a considerable source of raw Aetherium in Blackreach. 
An alliance between four Dwemer cities of Arkngthamz, Raldbthar, Mzulft, and Bthar-zel (now known as Deep Folk Crossing) was created to oversee the extraction, processing, and study of the rare crystal. 
The city of Arkngthamz led the project and served as the primary command and research center, while the city of Raldbthar, which happened to be located directly above Blackreach, dealt with the mining operations. Mzulft was used as a storage site, but it is unknown what role Bthar-zel took. The Aetherium Forge was constructed under the city of Bthalft in order to smelt it under precisely controlled conditions. 
The research into the crystal and the items it produced by the forge was a major success. The items produced by the Forge were artifacts of immense power, imbued from the moment of their creation with powerful enchantments. The dwarven alliance shattered almost immediately, as the four city-states and their rivals attempted to claim the Forge. The conflict later became known as the Aetherium Wars and it lasted for several decades. Years of internal disputes and infighting decimated the Dwemer states and was the primary reason for their sudden collapse, which in turn paved the way for High King Gellir's subsequent conquests. 
- The Elder Scrolls V: Dawnguard (First appearance)