Silicon

Silicon is the element in the periodic table that has the symbol Si and atomic number 14. It is the second most abundant element in the Earth's crust, making up 25.7% of it by mass. Silicon is the principal component of most semiconductor devices and, in the form of silica and silicates, in glass, cement, and ceramics. This abundance it part of the reason why the Urbankans came to Earth. (DW: Four to Doomsday)

Silicon-Based Life
Though not as common as carbon-based life, there are some silicon-based species in the Universe. Silicon-based life generally looks like rock, is very tough and breaks down into sand when it dies. According to Article 7954 of the Galactic Charter, Silicon-based life-forms must not be moved from their planet. (DW: The Stones of Blood)

Silicon-Based Lifeforms

 * Chalderans
 * Kastrians
 * Ogri

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.