The three main categories into which geologists classify rocks are:

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

2. Sedimentary rocks: Formed from the accumulation and cementation of sediments (fragments of other rocks, minerals, or organic materials).

3. Metamorphic rocks: Formed when existing igneous or sedimentary rocks are transformed by heat, pressure, or chemical reactions.