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

2. Sedimentary Rocks: Formed from the accumulation and cementation of sediments (rock fragments, mineral grains, and organic matter). Examples: sandstone, limestone, shale.

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

These three classes are the foundation of understanding rock types and how they fit into the rock cycle.