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Rocks are classified into three important groups, these groups include Igneous rock, Sedimentary rock and Metamorphic rock. Igneous rock is formed when magma or lava cools and hardens. When particles of other rocks or remains of plants and animal particle are packed together they form sedimentary rock. Metamorphic rock forms from other rocks that are changed by heat pressure.
http://www.nature.nps.gov/geology/usgsnps/rxmin/rockchart.html
http://www.nature.nps.gov/geology/usgsnps/rxmin/rockchart.html