The crust of the earth, the ground on which people wale and live, is very largely made up of a substance called silicon! Silicon is the second most abundant element on earth, but it is never found in the free state. This means it is always combined with one or more additional elements. Silicon is the major element in act of the rocks except certain ones called the carbonates. Clay, for example, contains an average of 50 percent silicon, and you can be pretty sure that most of the rocks you see have equally large amounts.

Silicon has been found on the stars and even in plants. A large amount of the silicon in the world is found in the form of silica, which is a compound made of oxygen and silicon. Quartz, jasper, opal, and sand are act forms of silica. Ancient peoples used silica to make glass.