A rock is a substance that occurs naturally and composed of grains of minerals and mineraloid matter. Grains of minerals are crystalline solids which form chemical compounds arranged in an orderly manner. Rock is categorized by chemical composition and how it’s formed. Most rocks contain silicate minerals and account for about one-third of all known mineral species and about 95% of the earth’s crust. Rocks from the earth’s outer crust, and earth’s outer solid layer. The proportion of silica in rocks and minerals is a major factor in determining their names and properties. Rocks are classified according to characteristics such as mineral and chemical composition, permeability, the texture of the constituent particles, and particle size. These physical properties are the result of the processes that formed the rocks. Over the course of time, rocks can transform from one type into another, as described by a geological model called the rock cycle. This transformation produces three general classes of rock: igneous, sedimentary, and metamorphic.