Rocks are aggregates of homogeneous substances called minerals.  Some rocks, for instance limestone, consist of a single mineral only but the majority consist of several minerals in varying proportions.

There are three basic types of rock:

1. Igneous: these are rocks that solidified directly from molten silicates, which geologists call magma. Igneous rock is formed by magma (molten rock) being cooled and becoming solid. They may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks
Examples are: granite, basalt, pumice and flint (which is a form of quartz).

2. Sedimentary: these are formed when igneous rocks are eroded as a sediment under the sea. Sedimentary rock is formed by deposition and consolidation of mineral and organic material and from precipitation of minerals from solution. The processes that form sedimentary rock occur at the surface of the Earth and within bodies of water.Fossils are often found in this layer.
Examples are limestone, chalk, sandstone.

3. Metamorphic: Any of a class of rocks that result from the alteration of preexisting rocks in response to changing geological conditions, including variations in temperature, pressure, and mechanical stress. The preexisting rocks may be igneous, sedimentary, or other metamorphic rocks.
Examples are: slate, marble, quartzite.”