Obsidian

The black stone known as Obsidian is the bane of magic, totally refractory and inactive. In instances such as the Obisidian Hall of Altdorf, Wizards can feel their power dimmed by the presence of so much while all around them.

Source

 * : Warhammer Armies: The Empire (6th Edition)
 * : pg. 54

[[Category:O]]