Magnesium

Magnesium was an element that "burns". When science teacher Ian Chesterton taught this to his Coal Hill School class, Time Lord student Susan Foreman, bored with this rather obvious "revelation", had in mind to make him burn too with one of her grandfather's everlasting matches. (PROSE: Time and Relative)

Magnesium was a vital nutrient and was necessary for living things to grow. (TV: The Waters of Mars)

On an Earth-like planet, the First Doctor saved John from a hostile native tribe by throwing magnesium powder on their fire, frightening them off with his "magic". He later used some to drive off a crocodile. (COMIC: The Didus Expedition)