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Rocks are naturally occurring solids composed of minerals or mineraloids. They form through geological processes and are found throughout Earth's crust and on other planetary bodies. Rocks are traditionally grouped into three main types by origin: igneous, sedimentary, and metamorphic.

Igneous rocks crystallize from molten material. Intrusive (plutonic) rocks cool slowly underground and typically have visible crystals, as in granite or diorite. Extrusive (volcanic) rocks erupt at the surface or solidify quickly, yielding fine-grained or glassy rocks such as basalt, andesite, rhyolite, obsidian, and pumice. They vary in mineral composition from felsic to ultramafic.

Sedimentary rocks form from weathered and transported fragments (clastic) or from mineral precipitation. Clastic examples include sandstone and shale. Chemical and biochemical rocks form by precipitation or accumulation of minerals, such as limestone (calcium carbonate), rock salt, coal, and chert. Sedimentary rocks often preserve fossils and record environmental conditions.

Metamorphic rocks arise when existing rocks are altered by heat, pressure, or chemically active fluids without melting. This can produce foliated rocks like slate, phyllite, schist, and gneiss, or nonfoliated rocks such as marble and quartzite. Metamorphism is common in mountain belts and near igneous intrusions.

The rock cycle describes the interconversion of rock types through tectonics, weathering, erosion, burial, and melting. Studying rocks helps reveal Earth’s history, resource distribution, and geologic processes that shape landscapes.