|Rock is a substance composed of minerals and classified according
to mineral composition. Rocks are generally classified by the processes
that formed them, and are thus separated into igneous, sedimentary, and
metamorphic rocks. Igneous rocks are formed from molten magma, sedimentary
rocks by deposition and compression of particulate matter, and metamorphic
rocks by either of the first two categories after being changed by the effects
of temperature and pressure. In cases where organic material leaves behind
an imprint of itself in rock, the result is known as a fossil.