Shadow Metal

Shadow Metal (不在金属(シャドウメタル)) is a rumoured metal which doesn't exist naturally in the world.

Principles
According to Saten Ruiko, rumour has it that when hundreds of esper abilities clash (like during the Daihaseisai), a metal is created which doesn't naturally exist.

Daihasei Festival Arc
During the Daihaseisai, Saten Ruiko, having learnt about the rumor concerning Shadow Metal, tried to investigate it, getting into trouble in the process. At that time, the rumor was described as having developed from the increased security to prevent people from stealing DNA maps during the games, apparently baseless.

However, during the Level 6 Shift attempt on Misaka Mikoto and the efforts of Kamijou Touma and Sogiita Gunha to save her, a metallic substance was formed from the clash between Mikoto and Gunha's powers when he made a path for Touma to get to Mikoto. After the experiment was stopped, they all take notice of the area now being completely covered with a strange metal coating.