A nonmetallic element that is the second most abundant on Earth, being exceeded only by oxygen. Symbol, Si. Silicon is not found free in nature, but occurs as the oxide and silicate. Sand, quartz, rock crystal, amethyst, agate, flint, jasper, and opal are some of the forms in which the oxide appears. Hornblende, orthoclase, kaolin, and biotite are a few of the numerous silicate minerals. Used in the electronics and space-age industries; used to make concrete, brick, and glass. Miners often develop a serious lung disease, silicosis, from breathing large quantities of the dust.