Silicon is named from the Latin silicis, flint and was isolated in 1824. Silicon makes up 25.7% of the earth's crust by weight and is the second most abundant element after oxygen. Silicon is not found free in nature but occurs as the oxide, and as silicates. Sand, quartz, rock crystal, amethyst, agate, flint, jasper and opal are some of the forms in which the oxide appears. Granite, hornblende, asbestos, feldspar, clay and mica are a few of the numerous silicate materials. Silicon is one of man's most useful elements. In the form of sand and clay it is used to make concrete and brick; it is a useful refractory material for high temperature work, and in the form of silicates it is used in making enamels, pottery, etc. Silica, as sand, is a principal ingredient of glass, one of the most inexpensive materials with excellent mechanical, optical, thermal and electrical properties. Silicon is important in plant and animal life.