1. Igneous rocks: Formed from the cooling and solidification of molten rock (magma or lava). Examples include granite, basalt, and obsidian.

2. Sedimentary rocks: Formed from the accumulation and cementation of sediments, such as sand, mud, and shells. Examples include sandstone, limestone, and shale.

3. Metamorphic rocks: Formed when existing rocks (igneous, sedimentary, or other metamorphic rocks) are transformed by heat, pressure, or chemical reactions. Examples include marble, slate, and gneiss.