Rocks are classified into three main types based on how they are formed: igneous, sedimentary, and metamorphic.

Igneous Rocks: These rocks are formed from the solidification of molten material, either magma (below the Earth's surface) or lava (on the Earth's surface). Igneous rocks can be further categorized into intrusive (formed below the surface, with slower cooling) and extrusive (formed on the surface, with rapid cooling). Examples include granite, basalt, and obsidian.

Sedimentary Rocks: These rocks are formed from the accumulation and compression of sediment, which can be derived from the weathering and erosion of pre-existing rocks, organic matter, or chemical precipitates. Sedimentary rocks are often found in layers or beds. Examples include sandstone, limestone, and shale.

Metamorphic Rocks: These rocks are formed from the alteration of pre-existing rocks due to high temperature, pressure, or chemical processes without completely melting them. This results in changes in mineralogy, texture, and sometimes the formation of foliation (layering). Examples include marble (from limestone), slate (from shale), and gneiss (from granite).

Each of these rock types has its own unique characteristics, formation processes, and uses.