Shadow Metal

Shadow Metal is a rumored metal which doesn't exist naturally in the world.

Principles
According to Saten Ruiko, rumor 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, Gunha took notice of the area was now covered with a strange metal coating.

Later, Aleister Crowley retrieved the Shadow Metal since he watched their fight through UNDER LINE.

Tokiwadai Election Arc
Following the events of the Daihaseisai in September and by the time of Tokiwadai's student council election in October, there had been an increased movement by a group connected to Academy City's previous generation, who used two squares (actually cubes) as their symbol, and apparently sought Shadow Metal, said to exist in the form of cubic crystals.

During the events surrounding the election, Kouzaku Mitori infiltrated a facility which turned out to house one of two cubes referred to as the Relic, which spoke to her and introduced itself to her as the previous #1 Level 5. Sometime afterwards, an influenced Kouzaku was heading for Tokiwadai with the cube in her hand, and a number of security robots encased in a mound of what appeared to be Shadow Metal behind her.

When Kokuchou Mayui's Relic manifested Synthesize Grid in response to Misaka Mikoto's approach, it started to produce Shadow Metal, a result of the grid consuming esper power.