* Igneous rocks: Formed from the cooling and solidification of molten rock (magma or lava).

* Sedimentary rocks: Formed from the accumulation and cementation of sediments (like sand, mud, or shells).

* Metamorphic rocks: Formed when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are transformed by heat and pressure.

So, the origin of the rock is the primary factor used to classify them.