The major classification of rocks is based on how they are formed, and there are three main types:

1. Igneous Rocks:

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

* Examples: Granite, basalt, obsidian, pumice

2. Sedimentary Rocks:

* Formation: Formed from the accumulation and cementation of sediments (rock fragments, minerals, organic matter) that were transported and deposited by water, wind, or ice.

* Examples: Sandstone, limestone, shale, coal

3. Metamorphic Rocks:

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

* Examples: Marble, slate, gneiss, quartzite

This is the basic classification of rocks. There are many sub-categories within each type based on their mineral composition, texture, and other characteristics.