Sulphur

Sulphur was an element.

In 1666, a number of villagers were burning sulphur in a forest where the Doctor's TARDIS landed. (TV: The Visitation)

The planet of the Gonds had an atmosphere that was high in both sulphur and ozone. (TV: The Krotons)

The Eighth Doctor used a sulphur-match to start a fire on the House of Zarn. (PROSE: The Eight Doctors)

In the Victorian era, the London fog was yellow and foul-smelling due to its sulphur content, caused people using coal fires to keep warm. (PROSE: The Bodysnatchers)

In 2003, Becky Lee Kowalczyck discovered sulphur deposits in the alley where the Eighth Doctor and Alistair Gordon Lethbridge-Stewart had been attacked by demons the previous night. (AUDIO: Minuet in Hell)