Silica | Definition & Facts | Britannica
alumina zirconia silica. silica, compound of the two most abundant elements in Earth 's crust, silicon and oxygen, SiO 2. The mass of Earth's crust is 59 percent silica, the main constituent of more than 95 percent of the known rocks. Silica has three main crystalline varieties: quartz (by far the most abundant), tridymite, and …