Silicon
Silicon (microelectronics) is a chemical element (Si). It is a quite common element (the eighth most common element) in the universe and widely found in dusts, sands etc. But it is rarely occurs as pure element in nature. It is the principal component of most semiconductor devices
Silica
Silica (Silicon dioxide) is an oxide of silicon with the chemical formula SiO2. It is quite commonly found in nature as sand or quartz.
Quartz glass
When silicon dioxide is cooled rapidly, it does not crystalize but solidifies as a glass.
Silicones
Silicones (high performance polymer) are polymer that include silicon together with carbon, hydrogen, oxygen and other chemical elements.
Optics 