Rocks
mainly consist of minerals, which are crystalline solids made up of atoms chemically
bonded together. Some rocks are also made up of mineraloids, which are
mineral-like substances lacking crystalline structure. A rock's mineral
composition is determined by how it was formed. Approximately one third of all known mineral species are
silicates, and silica makes up about 95% of the earth's crust. The proportion
of silica in rocks and minerals is a major determinant of their names and
properties.
Classification of rocks is based on characteristics such as mineral composition, permeability, texture, and particle size. The processes that formed the rocks were responsible for these physical properties. Rocks can change from one type to another over time, as described by a geological model called the rock cycle. There are three general classes of rocks produced by this process: igneous, sedimentary, and metamorphic.