Silicon

The abundance of silicon was one of the reasons the Urbankans came to Earth. It was one of the minerals they planned to take. (TV: Four to Doomsday)

Silicon-based life
Though not as common as carbon-based life, there were some silicon-based species in the universe. Silicon-based life generally looked like rock, was very tough and broke down into sand when it died. According to Article 7594 of the Galactic Charter, silicon-based life-forms must not be moved from their planet. (TV: The Stones of Blood)

In 1903, after receiving a wealth of information from the future, Grigori Rasputin foresaw, among other things, creatures made of silicon. (AUDIO: The Wanderer)

Silicon-based lifeforms
Kastrians were silicon-based. They were empowered by irradiation, an ability that allowed them to be restored to life if a single particle remained in existence. The genetic codes that permitted this were kept in special rings. (AUDIO: Eldrad Must Die!)

Other silicon-based lifeforms were: the Ogri, (TV: The Stones of Blood), the Ogre of Hyfor Three (PROSE: The Stone Rose) and the Chalderans. (COMIC: Under the Volcano)

Behind the scenes
Silicon-based life is found in other science fiction, owing to the fact that carbon and silicon form the same number of molecular bonds with other elements. Silicon-based life is scientifically possible, but no real-world examples have been found.