Silicon

Silicon was the element in the periodic table with the symbol Si and atomic number 14. It was the second most abundant element in the Earth's crust, making up 25.7% of it by mass. Silicon was the principal component of most semiconductor devices and, in the form of silica and silicates, in glass, cement, and ceramics. This abundance 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.