Silicon is classified as a metalloid element because it has properties of both metals and nonmetals. As the second-most abundant element in Earth’s crust — second only to oxygen — it accounts for more than 25 percent of the crust by weight. By mass, silicon is the eighth-most common element in the universe. Yet it is rarely found free in nature. Rather, it occurs chiefly in silicate minerals, such as feldspars, hornblendes, phyllosilicates, the serpentine group of minerals and micas, and in quartz and microcrystalline forms of quartz, such as agate and flint. These minerals are found in many different rocks, including granites and schists. Sand is commonly composed of small particles of quartz.