Quartz
The purest condition of silicon is that of quartz, in which it forms hexagonal crystals terminated by six-sided summits. It is a very abundant and widely diffused mineral. Quartz rock is mainly composed of it, and it is in this substance that gold is more frequently found than in any other. Quartz is the principal constituent of nearly all granites, as well as the numerous sandstones, limestones, trap-rocks, etc. Sands of both desert and sea-shore, and the common flint, are chiefly composed of it; and many stones, as the agate, amethyst, carnelian, chalcedony, jasper, rock-crystal, etc., are simply varieties of quartz. It is thought that a great deal of the silica which exists in nature has been originally deposited in the soluble condition. The structure of the chalcedony, etc., proves that they were formed by a solution of silica having penetrated into a cavity in the surrounding rock and there crystallized. See throughout this work for a description of the substances mentioned.