Rocks are naturally-formed aggregates of minerals that make up all of the Earth’s crust and the solid parts of the mantle below. Most are composed of several different types of mineral, though many of the rock-forming minerals are silicates. The physical characteristics of a rock are determined by the chemistry of its mineral constituents and by the processes that it endured during and since its formation. Crystallized in its structures, therefore, is a wealth of information about the conditions that once existed on the Earth’s surface, and those found today at great depths within the planet. There are three types of rock – igneous, metamorphic and sedimentary – though as with all classifications, these divisions can be arbitrary.