Vastial was a white powder which could be used as an explosive. Vastial was harmless at lower temperatures, but at 10 degrees centigrade, it became unstable and at 15 degrees would combust. The resulting explosion could injure Cybermen. Vastial could be found on the planet Telos. It was used by Cryon rebels against the Cybermen who had invaded Telos. (TV: Attack of the Cybermen)