Chronosteel

Chronosteen was a metal that could only be forged within the Time Vortex. Because of this, even a small piece could change the course of history. Three pieces of Chronosteel at different points in the history of Earth had the potential to cause the entire planet to be sucked into the Time Vortex. Once the pieces were brought together in the same time period, the danger passed. (TV: Lost in Time) This two part episode was part of The Sarah Jane Adventures