Rock

An aggregate of one or more minerals; or a body of undifferentiated mineral matter. The three classes of rocks are: (a) igneous - crystalline rocks formed from molten material (e.g. granite and basalt); (b) sedimentary - resulting from the consolidation of loose sediment that has accumulated in layers (e.g. sandstone, shale and limestone); (c) metamorphic - formed from pre-exisiting rock as a result of burial, heat and pressure.