Quartz

Quartz or PJX1-8 was a crystalline mineral abundant on the planet Earth, but rarer elsewhere. (TV: The Pirate Planet)

The Tenth Doctor explained that its name was derived from the German word "quarz," which originated from the Slavic meaning for "hard". (PROSE: The Bog Warrior)

Quartz was frequently used for decorative purposes but its electrical properties made it valuable in electronics. It was often used as a component of clocks. (TV: The Sontaran Experiment) Quartz arranged in regular patterns within rocks could form circuit boards. (PROSE: The Deviant Strain)

In 2013, Eldrad's presence in Ambermouth prompted the growth of quartz to the point where the water in the harbour was solidified. Bird and fish populations fell dramatically as they succumbed to infections. The local community, as well as the Fifth Doctor, Nyssa and Turlough, were all infected. This resulted in the hardening of their skin as it became covered in the crystals, with later stages seeing their arms become weaponised. After Eldrad's ring was destroyed, along with two incarnations of Eldrad, the crystals receded. (AUDIO: Eldrad Must Die!)

Quartz eyes were set from behind in the faces of the Cave of Five Hundred Eyes in China. They reflected light to give the appearance of movement but could also be removed to look through. (TV: Marco Polo)

The first Dalek Emperor, upon taking the title, ordered the gathering of quartz for the creation of his special casing. (COMIC: Genesis of Evil)