Silicon

Silicon is a common element present in stars and is a principal component of meteorites known as aerolites. It is also a component of tektites, a natural glass of uncertain origin, but believed by some to be ejected lunar material from Tycho crater (the current site of Tycho City). Along with germanium, it was a primary component in the production of transistors. Silicon makes up 25.7 percent of the Earth's crust, by mass, and is the second most abundant element, being exceeded only by oxygen.

Lifeforms
The Organians watched a planet for ten thousand years, because it contained a silicon-based virus. They used the reactions of species that encountered it to determine the level of intelligence of that species, because carbon-based lifeforms cannot naturally defend against silicon. Among the species which the Organians observed with this silicon virus were Klingons, Cardassians and Humans. 

Silicon-based lifeforms were considered total fantasy by the Federation until the discovery of the Horta on Janus VI on stardate 3196. A silicon-based lifeform cannot survive in a Class M atmosphere for more than brief periods of time. They are also much less susceptible to phaser fire than carbon-based lifeforms; a shot at full power from an adapted type 2 phaser is capable of only mildly damaging one. 

Another at least partially silicon-based lifeform was the microbrain of Velara III. It consisted of several inorganic elements and compounds, including silicon. 

Related links

 * Bioactive silicon
 * Disilicon polymer
 * Polysilicate verterium
 * Silica
 * Silicate
 * Silicon-based liquid
 * Silicon-based parasite
 * Silicon nodule
 * Thermal concrete